Converting d20 to single roll d%

[marli]

I'm building. A rpg system and am currently have a magic system based on d20. However I've converted combat and combat and skills system to d% to speed up in game usage while., so want to change magic to match.
Currently the system works as below each player has between 1 and 35 mana points per adventure average is about 5 to 10.
to cast a spell a player chooses to spend 1 to 19 points.
They they then have to roll equal or under to cast the spell.
They can repeat the roll as many times as they like to increase the power of the spell.
If they roll a 18 they suffer 1 wound.
If they roll an 19 the spells target is redirected by the DM (initally favourable but collect to many 19s then the targeting gets more and more negetive-eg non enemies, innocents, allied.,)
rolling a 20 and the spell fails and the caster takes 10 wounds.

Rolling 18 or 19 AND rolling equal or lower to your target does not mean you cannot roll again
if you roll a 20 on your first roll nothing happens.

Now how would you convert this to a single d% roll?

I have some ideas which I'll post later to not distort other people's thinking

 

[marli]

I'm thinking going for the minimum power level of a spell mean there is no feedback risk!???

 

[Darkness]

Now how would you convert this to a single d% roll?

When converting d20 to d100, you'll probably want to multiply by 5.

If they roll a 18 they suffer 1 wound.
If they roll an 19 the spells target is redirected by the DM (initally favourable but collect to many 19s then the targeting gets more and more negetive-eg non enemies, innocents, allied.,)
rolling a 20 and the spell fails and the caster takes 10 wounds.

Rolling 18 or 19 AND rolling equal or lower to your target does not mean you cannot roll again
if you roll a 20 on your first roll nothing happens.

18 = 86-90
19 = 91-95
20 = 96-00

18-19 = 86-95

 

[marli]

Ah seems there is some confusion about what I want to achieve.
I want these multiple rolls converted to a single roll

 

[Starfox]

The only method I can see is to make all the probability calculations and then make a table out of that. But in my mind that loses some of the charm of the system; that the player has to nerve-wrackingly continue to bet on his powers. Its like raising the bet in poker - the potential win increases, but so does your part of any loss.

Overall, this system seems very fragile in that magic become too chancy for everyday use.

 

[marli]

Damn! Post failed to post.
Anyway that what I really want to convert, the risk elements, I have multiple solutions but none really convert this feel.
The whole point of magic In my system is that it's frowned on by the general population and outright banned in some places.

Why I want to use d%

With a d% you get lots of results of a single roll
You get d% of course
you get double digit d10(which relates closely to the d%)
you get the single digit d10
you two d10s to compare (matching, higher, lower)
you also get reverse percentile (31 and 39 are become 13 and 93)

It gives you a lot of maths to work with to produce a fast system.

 

[underfoot007ct]

Damn! Post failed to post.
Anyway that what I really want to convert, the risk elements, I have multiple solutions but none really convert this feel.
The whole point of magic In my system is that it's frowned on by the general population and outright banned in some places.

Why I want to use d%

With a d% you get lots of results of a single roll
You get d% of course
you get double digit d10(which relates closely to the d%)
you get the single digit d10
you two d10s to compare (matching, higher, lower)
you also get reverse percentile (31 and 39 are become 13 and 93)

It gives you a lot of maths to work with to produce a fast system.

I understand you want to use d100, but with all these items above, how & why do you think it this would be faster than roll a d20 & add a bonus? How is "lots of math" fast ?

 

[marli]

You don't. My objective is to do most maths as part of the game design. For example there a 10% chance of a 5 on a dice and a 19% chance on either. So I can create rules like 5s on either dice causes x, effect on top of the % success or fail. The effect could be inclusive or exclusive of the success or the failure.
The point is the large amount of mathematical possibilities means a d% should give me enough targets to choose from.

 

[Dannyalcatraz]

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Reported.

 

[marli]

Ok seems I cont peak anyones interest in this matimatical conundrum
I see three core solutions, however each of them has issues.
1)choose spell level, roll lower that X.
this has the disadvantage X would get smaller as the level got higher, meaning the production of this X would be calculated and most likely need a table for the player to consult. it also means that successes would have an increased chance of producing ever lower dice rolls as level increased, meaning relating the numbers on the dice to effects would decrease not increase.
however since we are creating a table the max spell level colud be alot higher.(some of my tests produced clean progressions up to 18 levels and vialbe table up to 27 levels)
negetive effects would result from rolling certain specific low numbers.(eg spell level)

2)choose a spell level roll higher then X
this has the advantage that X can be easily found from target level.
eg
lvl1=roll above 11
lvl9=roll above 99
such a simple system however restricts the max spell level to 10(0 to 9)
it does have the advantage that high level successes produces higer dice rolls, meaning negetive effects could be directly taken from the dice.

3)compare the two dice, ive not thought this through properly
but somthing like one dice must be higher than another?????(auto 55+% failure ) equal while </> X???
not equal and both higher then the spell level????