Hi Urban - can't answer your question precisely but this may help.
I also use an 8x10 print drum and have modified it as follows.
Firstly two white plastic ribs glued in place, opposite each other, that run from top to bottom. This allows for either one 8x10 sheet to stay in place, (or Whole Plate which was what I was using) or two 5x7 sheets, or four 4x5 sheets. The ribs were pillaged from the edges of some plastic electrical ducting and glued in with Araldite.
If loading 4x5, I load two to the bottom of the tank, sideways, and then insert this space cut from a plastic lid, though a CD would do fine if you can cut it safely. This prevents any risk of the sheets sliding around and overlapping.
Then you can load the other two on top of the disk.
Incidentally the fine ribs you can see on the inside, (in black) are to help keep the back of the paper or film away from the drum surface - helps with clearing any anti-halation coating. Does yours have these?
I have an electric roller and use about 250ml of liquid. I could probably use less but that's easy to measure. Works like a dream for 4x5 film, though I do adjust the chemistry a little - I add 25% plain water, (from 200ml to 250ml) and reduce dev time by 15% since it's a continuous rolling.
That might help. Maybe.
