Shelly, I dont know but I seriously doubt Steve recommends adding 20 to 30% development time just because you switched to 120 film from 35 mm. I see no reason to do this nor have I heard anybody recommend this before. Perhaps you misunderstood?

But anyway, since you shot some at asa 100 and some at asa 50, I would recommend to develop as if all the roll was shot at 100, is better to have some overdeveloped negs than underdeveloped.

The 15 to 20% rule of thumb is ok, but given the vagaries of film developing and all the other factors that affect expsoure perhaps 50 asa is the correct EI for you, if so then your negatives shot at 100 would be underexposed and underdeveloped. Not a good thing! You can always give your negatives more printing time, but you cannot get info that is not in the neg.

So to answer your question, I see no reason to give 20 to 30% more development time just because you moved up to 120. If you are developing with single reel tanks and you fill it so that it covers the reel, the developer there should have enough strength to develop your film in the same time as 35 mm. For the case of the apx film I would say develop it just like all was shot at 100 asa and see what are the results before you go making changes in your usual development routine.