I did the non mathematic way. to avoid the reels from getting wet, I put them in, and set the plunger up to the appropiate height, removed those reels, and put in a bunch of 35mm reels until it reached the height of the drum, poured water in til it was full, then poured it back out to find it was about 6 cups for the (3) 120 and (1) 35mm reel, so took 3 cups converted to milileters (709ml approx) , figured 750ml was fine ( was dead to the middle almost), so ya that works