If you use something like BTZS tubes that permit use of minimal amounts of chemicals, you can use the following rules of thumb to determine how much HC-110 dilution B needs to be used:
1) for 120 film, use an amount that is the greater of:
a) the amount needed to cover the reels in the tank, or
b) 192 ml (200 ml is what I would use); and
2) for 4 x 5 sheet film, use an amount that is the greater of:
a) the amount required by the tanks/tubes/reels to ensure all of the sheets are in contact with or covered by the developer, or
b) 48 ml (50 ml is what I would use) times the number of 4 x 5 sheets being processed.
In most cases it is the requirements of the tanks or tubes you are using that set the minimum volume, but not always.
You shouldn't have to make up separate amounts of stock for 120 and 4 x 5. You will, however, most likely need to use different amounts of working solution because of the differing amounts required by the different tanks and reels or tubes.
If you buy the MOD insert, you can of course use the same Paterson tank and Paterson reels to develop your 120 film.
Good luck, and I hope you have fun!