I'm not sure of a location in the code that specifically says it, but if you read through the commentary and supporting documentation, you'll get a good feel for where those numbers come from. That, in turn, will give you the background necessary to make the necessary judgement call.

I'm guessing most of your issue is with the side and leeward walls, correct? If the 30' tall portion is relatively small in area, then I'd be inclined to use the pressures using h=120'. If it's relatively large in plan (say, greater than 30x30), then I'd probably consider leeward forces for h=30' when you have both windward and leeward 30' walls. If you have a windward 120' and leeward 30', use h=30'.

It mostly boils down to wind flow separation. If the primary flow separation is being caused by the 120' section, use h=120'. If it's being caused by the 30', use that. If you're not sure, then fail conservative and assume the larger load of the two.