The original poster is trying to get it to taper. The way I interpret the title and first post is that he WANTS to lose boost (I know,I'm baffled too.) And as far as I understand the concept, the MBC can't do that.
A MBC on the right turbo can hit and hold a target boost. No big deal. But he isn't asking that. He want's the MBC to hit target boost, then slowly loose boost. I don't know why. Sounds silly to me. But I don't think it can be done at a pressure the turbo is capable of holding 'til redline, like 15 psi or whatever.
If he wanted to set the boost higher, above the PSI the turbo could hold with the wastegate line plugged, then sure, it'd have to taper off, like Timdog said in his first post. Set the boost for 20 (don't really! I'm just thinking out loud here.) And it'll hit 20 and come 4500 rpm or so it'll start falling off to it's happy-place at 15 psi or so.
So Blackie, if that is your real name... Why don't you tell us what you are working with (turbo, MBC, relevant mods) and what exactly you are trying to achieve. (Why do you want to taper boost off?)