The "reduce your charge by 15% to replicate a blackpowder load" thing scares people about 777. I've used it on and off for years, and honestly I don't see any reason to be nervous about the stuff. Choosing pyrodex or 777, I prefer pyrodex myself because under compression 777 gets kinda inconsistent. I don't have a chrono but the difference is audible...did it just go bang or did it go BANG! And its a little harder to light off than pyrodex, sometimes a #11 cap wont do it and I've had some hangfires with 777.
I can't suggest any charges for the 36 cal because I don't have one myself, but your math is right, 22grains of blackpowder reduced by 15% is 18.7grains. If you upped it to 20grains of 777 with your 5grain increment powder measure, you're not going to hurt anything. I say take 18 or 24 shots with 15grains of 777 and then clean up and take another 18 or 24 shots with 20grains of 777 and see if you can tell any difference in your groups...at the very least you have a reason to go make smoke!
Have fun and post pictures!