I'm not trying to be a smart guy, if you want to use that tank for 1 35mm reel. Just put the empty extra reel on top of your loaded reel and use 300mL of solution. I know Paterson made single reel tank for the original system. The little plastic doughnut ring for Paterson is supposed to hold a single reel down, I never used one.
Jobo reels really vexed me, until I learned the secret of snipping the tiny corners off the film.
Fresh Paterson reels and of course Hewes have always been the easiest for me to load.