There's no way of doing a Real Procs Per system on an on-hit basis like crits. Critical hits are a RPP system in and of themselves. An RPPM says, "On average, you'll get X number of procs ever minute". The mathematics of crit works the exact same way: "on average, you'll get X number of crits over Y number of attacks".
If you wanted to remove the RNG from it, you would have to do a flat % damage buff for every auto attack and critable ability. 30% crit would = exactly 30% more damage with no RNG involved.
Honestly, though, I don't think that would be a terrible idea. RNG is the bane of any competitive game. Once you introduce RNG into a competitive environment, it's no longer entirely skill-based as it should be. A single auto attack can determine a game based on whether or not it's a critical hit. It doesn't leave anything to skill, it's essentially a flip of a coin in whether or not you win at that point.
Granted those kinds of moments are inappreciable over the larger scope of the game, but their existence still points out a fundamental flaw in the competitive environment.