So i have some doubles that have 32% and only 1300psi left. I can fill air for free at my fire dept. So I usually just top off then check O2 if i dont need another nitrox fill. However this week im planning a dive that would be ideal to have 29% and doesnt need that many cubic feet, so i thought it would be nice to just fill until i was at 29% and stop regardless of final pressure. I am comfortable with the math to partial pressure blend using 100% 02 topped off with air, and i can easily figure what a resulting mix would be if topping off a nitrox mix to a known pressure. But for whatever reason i cant seem to figure out a direct equation to tell me at what pressure to stop when diluting existing nitrox to a desired mix. I have come to an answer after "plugging and chugging" and know i need a fill to 1800psi, but would love it if someone could explain a straightforward mathematical way to get directly to this number. Thanks in advance.