im looking at the monk capstone with its +10 con and +1 passive ki regen up to your concentration.
Honesly it does seem to be worth taking at 20 even if i spam all my abilities i have way more ki than i need in fire stance anyways. If i run out of ki then i just wait 5 or so seconds and i even have enough ki for another tod if needed. At the moment i have getting +1ki regen from ninja spy anyways. Honeslty the monk Capstone seems very lackluster compared to the other class capstones. Would be nice if the capstone did something different depending on what stance you where in. As is its not realty worth taking imho