I think I can answer at least one of polyglot's questions .
The 120 and 220 spools are identical. If they weren't, you wouldn't be able to use the empty spool from a just exposed 220 roll as your next take-up spool.
And proceeding now to some guesses....
There is a small amount of excess film on each end of a 120 roll. If you double the length of a 120 roll, you will end up with twice the amount of that excess - I would guess that you only need that excess once
And as for the length of the leader and the trailer, you can probably use the same lengths of "leader and trailer as a roll of 120 has.