I guess I've just been doing this for so long it's become second nature for me. Once I know the "detonation" point I can usually pretty much eyeball the board (which is marked in 10' squares) and say right away who's in or out; and if there's one or more creatures close to the edge I'll get more finicky and if that doesn't help I'll leave it to the dice.
As for whether someone near the edge is in or out, I'll just give a big bonus on the saving throw and if the bonus gets you to 20+ then you were outside the area and take no damage. Obviously the same goes for the opponents. (your example reverses the mechanics, using the 4e method of attack for damage rather than defensive saving throws).
OK, well, my feeling is that these things tend to become focal points for arguments, because of course its a big deal if I take 10d6 damage or not! GMs tend to fall into camps. There's the 'hard ass' who rules with a bloody hand, the 'soft heart' who usually rules so that the PC lives, and the 'mechanist' who adds some die rolls and modifiers and may take 5 minutes to deal with one fireball.
Never mind that the idea of only one human-sized bipedal creature being able to occupy a 5x5' space is itself kinda silly unless said person is waving a very large weapon around.
Which is exactly when 4e says this, outside of combat there are no such rules. I'd note that many PCs can move THROUGH a square, and in some cases its possible to occupy the same space too, though its not common. I mean, sure, I can see a point that says "a halfling with a dagger and a wizard with a wand could probably stand back to back in a square". I've just seen so few, if any, situations where it MATTERED if it was one square or two that I find the whole thing academic. I'm highly into the 'what works in practice' camp.
A complexity I don't mind.
The trick is not to remember it all, but to remember where to look it up when needed. And, over the years I've gone through and rewritten absolutely every spell in the game, and put them online so they're easy for us all to look up.
Meh, I don't terribly mind looking some things up, but AD&D is a PITA! I mean, with 4e I barely HAVE to look anything up, and WotC designed the presentation so it would fit nicely into an online database which I didn't have to write! Frankly, D&D has one nice feature that has pretty consistently been true, its an easy game to reference. 5e TBH I find a bit of an exception here, its rules are not well organized.
Shrug - to me, that's just how the game is played.
And when the outcome is possibly the difference between life and death for your PC, wouldn't you want to dive into the minutae and make sure things were done right?
No, actually. I mean, I want it done in a way that is consistent with the way the game is intended to work. I'm fairly gamist and I enjoy exercising the workings of the game. Do I want to have to enforce rules on exactly how many arrows the orc fired before I got pigstuck? Nope, not really. If I invent some tactic that leverages "he's going to run out of arrows eventually" of course I want that to be feasible, but it can work on a check that represents how good I am at making such a plan, for example.
I don't deny it helps with agency. On the contrary, I think it helps too much.
My point is simply that if the PCs aren't in a position to know all the consequences then the players shouldn't be either, and thus there'll sometimes be some agency they just have to do without.
Right, this is the 'player test of skill' aspect of the original classic dungeon crawl, which is a very necessary part of that type of play (and when I say 'dungeon crawl' it can also encompass other similar kinds of exploration/looting situations, like hex crawls, certain kinds of intrigue, etc.). I think when it is projected beyond that, then it becomes an impediment. This is part of what is problematic with 2e.
Now, I don't mind surprising players, but I'm OK with them having knowledge that PCs don't. If they are playing to see what the PCs will do, then they're going to play in character, and it may be advantageous for them to know certain things in order to do that.
Something basic like swinging a sword at a foe: yeah, the usual consequences are obvious - you're either going to hurt the foe, kill the foe, or do nothing to the foe. There's also possible unusual consequences - you might fumble and do something you didn't want to do, or you might hit the foe so hard your swing follows through into something else (a.k.a. Cleave in 3e), or you might diasrm the foe, or trip it, etc., depending on system.
But something basic like going left instead of right at an intersection - assuming no pre-scouting or other foreknowledge - the PCs have no way of knowing what consequences will arise from that decision, and thus neither should the players.
I just don't see the value of a decision in which there's no real substance. I mean, again, this is an artifact of the 'explore the dungeon maze' paradigm, where the PCs may well choose left simply because it gives them a chance to fill in a part of the map and search for a suspected secret room or something. Anyway, at least there will be map consequences that may eventually matter down the road. In a narrative focus on characters its color.
Yeah, don't get me started on falling damage; it's bugged me since day 1.
Ditto with whichever boneheaded edition it was that gave the spell
Reverse Gravity a duration of 10 minutes. The idea, I'm sure, was that this would for 10 minutes just cause anyone entering the area (or the individual target, it's been done both ways) to crash to the ceiling. But what happens if it's cast outdoors where there is no ceiling? Think about it.....
A 10-minute upward "fall" doesn't quite put you into space, but you'll have long since suffocated by the time you come back down...and for the time you're falling upward you would always keep accelerating a bit, as terminal velocity is caused by air resistance which steadily decreases as you go higher. I really don't think they quite thought this one through...
LOL! Which edition did that? I think it lasts 1 segment in 1e, making it kind of a weak 7th level spell, actually. However, it has several interesting characteristics. The main one being it affects an AREA not a target, so there are no saves. Questioner of All Things used to memorize it as a fairly stock ploy to deal with nasty high level save problems. Usually you can get some advantage out of it. Overall a weak spell in that edition.
None of it is perfect, but if you take an average you'll at least have a guideline to start from - which is indisputably better than nothing.
Again, I think you're being a bit pessimistic. I mean, I'm sure I've got it down at least past the 99.6% point...
Lanefan
hehe, well, I could be a pessimist!

I mean, I did make a price chart. It was mostly just me fooling around though. I don't really intend to use it in a game, and its fairly arbitrary. However, it would work. Honestly the 1e chart is not terrible.