The stock fuel pressure regulator is rising rate, 1:1 ratio. When you upgrade it you just need to stay with 1:1. The key is that it is large enough, then it doesn't mess with your tuning by being inconsistent.

I think the e-Manage is a piggy-back controller (used in conjunction with the stock ECU)? Normally with piggy-backs you are limited to around 650cc injectors. "Pulling out" a lot of fuel to compensate for large injectors makes tuning diffuclt.
However, e-Manage might get around that by talking directly to the ignition and/or injectors.

For 400 WHP you might be looking at 1000cc injectors.

So if e-Manage can't handle that size, you'll need to switch to ECMLink or a full standalone.