So I did this a couple years ago, and if you are dead set on getting the MBC dialed in (weather dependent I might add) here is what I ended up doing...
I used a spring setup that was almost right at desired, and then used the MBC to dial it in the rest of the way. If I used a spring that was too heavy, I was unable to get it to crack open at the correct the time, and the MBC is only there to keep the wastge gate closed longer than the spring allows.
So.. used a spring that will allow the WG to crack earlier than you want, and then dial it in with the MBC... I was able to keep fuel rail pressure almost perfect, but run much higher boost than I would normally since I didn't deal with boost spikes...
This log is pretty gnarly... holding 16.5psi and almost perfect rail pressure even with the spikes... AJPTurbo tuned throughout this process..