Interesting, I recently learned about this.

So there is a term called "Full Kelly" which is were you actually wager the amount given by the formula. This is pretty aggressive and not for the faint of heart. Usually, you do something like a "half Kelly" where you halve the suggested bet amount. Usually because you assume you might be incorrect in your estimated advantage.

What I haven't found yet is some mental math rule to apply this. Given some assumed advantage, what percentage should I "wager"?

What bet you can make consistently gives you at least 51% odds? As far as I know, the best you can do in a casino is 48%. If I plug 48% into the simulation, it states there is no Kelly edge for that bet.

Back in the day when I played HU Hyper-Turbos on Pokerstars I created one of these in Mathematica (not as fancy with Monte-Carlo)

A similar calculator created many years ago is at https://www.albionresearch.com/tools/kelly