I am making a simple browser rpg game, similar to battleknight. To keep the game simple, I have just Strength, Constitution, and Luck. But as luck is random based, I'm struggling with fight calculations. I need some kind of formula to calculate attacks.

Preview from battleknight

UPDATE I was thinking about this model:

Strength - base dmg.

speed - chance of attack avoidance

toughness - increase HP

luck - crit. hit chance

defence - chance of attack blocking.

As I expect, there will be fight between high and lower ranked players so I want to calculate attack differently. My idea is sum of stronger player stats(this will be cap). So random based attributes will be divided by this sum. Example:

cap = sumOfStrongerPlayerAttributes
player1Luck = player1.luck / cap
player2Luck = player2.luck / cap
// also luck can be devided for ensure that crit hits will not be so often

Also crit. hit dmg. will be calculated from based dmg. like:

critHitDmg = player.strength * someKindOfConst

This model will prevent from too often crits. And player who will spend all his resources to level up luck, will have low stamina, defence or base dmg (which is also important for crit. hit dmg)