Overlap is your friend!!!!

Too much is bad, that's why you always want to shoot on a rising #, but when you get overlap nicely lined up it helps smooth out the curve. For example, with good overlap you might see PSs in the low 200s and a nadir in the mid 100s. Without the overlap, on a dose that gives you a good drop though, you might see that same nadir, but PSs in the low 400s instead. That means each shot is like starting over from scratch, and then you have big drops & steep climbs. That's why sometimes it makes sense to shoot early, so you can catch the #s before they get high and take advantage of more overlap than you might be seeing 12/12.
I haven't studied his #s at length, but on general principles I would say that if you tried 3.6 for a few days and it didn't seem to be enough, the next step would be to try 3.8. With the 4u, you perhaps saw a dramatic breakthrough - good, but probably a hair too much since you want a shootable # at +12. My experience with Bix was that only small changes were generally needed. I think you need to shoot what you feel safe with though. Personally, I would probably go back to 3.6 to regain my bearings and get some more mid-cycle tests in to be sure it wasn't giving him better results than you realized, and then if not, try 3.8.
For settling in period, ECID. Bix usually was about 3 cycles when I would see much better results, sometimes 4. I think 3 - 5 days is a fairly standard recommendation.
Since you got surprisingly good results these couple cycles on lower doses you could also try those a little longer and see how it goes. I wouldn't stay at lower doses too long though, especially since you have tried all those doses before (sometimes it just seems like he is married to those yellows, doesn't it? weird!) If you don't get improvement (or #s start getting higher) in a couple cycles, then you could jump back up to 3.6 or 3.8.
Personally, I probably would not shoot 4u unless you give 3.8 a chance for a few days and it is not enough. Although he still may end up needing 4u, you know that PS was too low, so that suggests to me that dose is too high for BID.