@tetrasodium You may or may not want to match 4d6-and-drop-the-lowest probability but, if you do, here're a couple of options for that:
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This is a very interesting implementation.
Basically, we assume an underlying distribution and we sample values from it, with the prescription that we also maintain the balance for the percentiles for all the scores in the array of the 6 abilities (if you have the 50+nth percentile on an ability you must have the 50-nth percentile on another ability), so that the final array has a guaranteed median of 12, the distribution is symmetric (within rounding precision), but the kurtosis depends on how much you min-max your choices.
Something interesting emerges if we look at the modifiers instead of the raw scores (which are pretty much never used in the game, except maybe str for carrying capacity). One thing intrinsic to how ability modifiers are computed since 3E is that odd scores are always "suboptimal" in the sense that an increase of 1 point from an even number doesn't give any mechanical advantage, while a decrese of 1 point results in a modifier reduced by 1.
With this sysyem the basic array would be a 12 in each score, for a +1 modifier in each, or a total +2 modifier for each pair of abilities.
If we use the first variant, and stick to even numbers, the "good" choices are 10-14 (we lose a +1 on one score and gain it on another one), and 8-16 (we lose/gain 2). All other choices are poor because we lose a +1 modifier on one side without gaining it on the other (see 11-13, 9-15, 7-17 and 5-18)
This however doesn't account for background modifiers, that allow us to add 1 point on a given score.
This is very useful, as we can "gain" modifiers for free on a single pair: if we add the free 1 point increase to the lowest score of any single pair, ALL choices become optimal again (we have a total +2 modifier on each pair), while if we add it to the highest score, instead, all choices are optimal except for the pair that would be 5-19, were we still have a total +1 modifier (also some DMs may not allow starting with a 19 in a score). In the economy of the game it probably still "sucks" to have odd ability scores, but at least you're not effectively penalized in terms of total ability score modifiers. In LU however single point increases can be bought with a modest investment in a stronghold, without resorting to magic items, so it may still be a good deal, especially for characters that start at level higher than 1 and/or have enough money.
The second variant removes the constraint on the symmetry of the distribution of the 6 ability scores and allows for more varied characters. Also interesting is that it is not possible to generate the standard array (or better, one ends up with a "credit" of 2 points that cannot be spent). The closest one could get with this system would be 15-14-13-12-12-8 (or 15-14-13-13-11-8), which are quite better as they replace a 12 and a 10 with either two 12s or an 11 and a 13. Again we have to factor in one "free" point coming from the background ASI.
In terms of modifiers, both options end up with a total of +6 (unless in the second case we bring an even number to and odd one, which would be quite masochistic IMO).
The most min-maxed options would be 18-18-18-5-5-5, which could become 19-18-18-5-5-5 or 18-18-18-6-5-5 after the ASI has been factored in. Again, the total modifier is +6 in the first case, and +7 in the second case (because we've been smart and put the free point in an odd score to gain a +1 modifier somewhere).
So, all in all, I think this could be dramatically simplified as follows:
- you can have the scores you want (no higher than 18 in each), as long as the sum of your modifiers is +6.
- you then add your background free ASI, which can bring your total sum of modifiers to +7 (but not always, one has to be a little bit smart to figure it out, no big deal though).
This dramatically simple approach can be tailored to more gritty or heroic campaigns simply by changing the total sum of modifiers (maybe +4 or +3 for more mundane campaigns, up to +8 for very epic ones), or by increasing/decreasing the highest score one can buy this way