What does Luck mean?
Luck could increase the chance of some combination of:
- Crit-ing (also potentially crit damage)
- Getting an item (if there are items)
- Having that item be a good item
- Success of any action you attempt
- Any random (unambiguously) good thing happening
- Any random (unambiguously) bad thing not happening
I'd suggest not using Luck for the first few in favour of having dedicated stats for those with less ambiguous name (e.g. accuracy / chance to hit, evasion / dodge, crit chance, block chance, drop rate, drop quality - dexterity has also been known to cover some combination of the first 4).
How do you calculate with Luck?
There are a few ways Luck can work: (can apply to other stats as well)
- Gives a chance of something happening (that wasn't possible before)
- Increases the chance of something happening (which is also be dependent on other stats)
- Make some non-percentage number bigger (e.g. crit damage)
The last one can simply be a multiplier (e.g. 1 Luck →
x *= 1.01, 2 Luck →
x *= 1.02 - it doesn't have to be 0.01 per 1 Luck).
For the first two you'll need to convert your Luck stat into a percentage (0%-100%) (there isn't a "correct" way to do this, you'll need to take into account the points mentioned below).
The second one is probably the most complicated, as you'll need to decide what exactly luck will change - I might suggest having it decrease the chance of failure: e.g. a 60% chance to hit with 50% luck gives you a
(1-0.6)*0.5 = 20% chance of missing (i.e. 80% chance to hit).
For any of them you'll need to consider a few things:
What kind of scaling do you want? In many games, stats use diminishing returns (grows fast at the beginning at then slows down) - this can make the game more balanced or easier to balance, as the stat bonuses are impactful at first, but don't get out of control.
Note that some linear increases may already give you diminishing returns - e.g. if your chance to hit increases from 25% to 50%, that's a 100% damage increase (you're hitting twice as often), but from 50% to 75% would only be a 50% damage increase. If you want to use a linear increase and the stat is unbounded, you'll need to figure out what will happen when you get to 100%.
What's the maximum value for the stat? Is there a maximum?
How much of a bonus would it provide at any point, especially compared to the same investment into other stats?
You may want to strive to have any given point provide a comparable benefit no matter which stat you throw it into at which point (with different stats changing your playstyle), or you could have some be significantly better (not one stat as a whole be better than another, but rather the m-th point of one be better than the n-th point of another), to allow for some min-maxing.
Should you include Luck?
Luck: (especially if it affects many things)
- Adds to the perceived complexity of the game by giving the player more things to put points into, which may be good or bad (this can be good if players can run out of others things to put points into, but this can be addressed in other ways)
- Can make the game harder to balance, unless Luck is mostly useless, in which case you should probably just remove it
- Can be hard for players to see the effects of, making players feel like getting more of it is a waste
- Probably wouldn't clearly and quantifiably explain what it does, making players less likely to want to invest in it
- Generally doesn't really affect a player's playstyle. As mentioned above, different stats could be used to allow the player to change their playstyle, but having one stat just sort of helping all over and not really have a clear focus goes against this.
Overall I'd say: no, you shouldn't include it, but you can possibly include more specific stats (as mentioned above).