You can model your situation at https://i-orp.com/Inflate/extended.html
It will take a few hours to read and understand all of the inputs and the outputs, but once you get your footing, you can operate the levers (change assumptions) and see what it does to your available spend. It considers SS, Roth converting, PPACA, and of course taxes. Those are not perfectly modeled, but very close in most cases.
What I've found out is that you can improve the spend a little bit, but by optimizing these kinds of moves isn't going to move you from Greyhound to Lear jet, hehehe!
One caveat I always say is to keep the model focused on taxes, and not on which tax buckets get better expected return rates, make the equity allocation the same across all tax buckets.