Trying to find best personal exposure T-MAX, can you help me decide which one it is?

I shoot portraits in a studio using a Mamiya RZ67 + single strobe setup. Before my next shoot, I wanted to find out what power my strobe should be set up to yield the highest quality negative/exposure I can get. Please see my results here:

T-MAX 400, D-76 1:1 developed normally at 10m15s, TF-4 fixer, lens aperture f/2.8 for all shots (only strobe power is increasing)
Full-res: http://i.imgur.com/AU0nJHz.jpg

The Sekonic L-358 metered reading is f/2.8 at ISO400 and 1/400. I feel as if I'm getting the most detail and contrast at f/8, while f/11 is getting washed out. However, because f/8 is +3 stops overexposed I'm not sure if my results are normal, and if you guys think that is the best exposure for me? I am also unsure if it would be okay to leave the lights at f/8 when switching over to Portra 400, though I read that Portra can handle +3 overexposure very well. Thoughts?

I think any of the 3 +1 to +3 are fine the question would be how you would like the result with portrait...

Portra does not have the same latitude as BW film. So mistakes in exposure are less recoverable. Also, while colors may be ok to priont with small over or under exposure, the colors do chamnge although you might not mnotice unless you compare exposure bracketed shots. That's something I did by bracketing like you did and then printing and/or scanning. You will see color shifts.

Also, I question whether your "normal"" exposure as figured by the light meter is actually accurate or that the developing and printing process is not causing changes in what is actually required. I'm not an expert on this; I don;t even do my own processing (a lab does). But I've seen these variances and even question whether my lab gets it right all the time. The other problem with negative BW or color film, is unlike chromes, you don't have a positive film result to compare the final print results with.

how are you processing your film?