Just had transom closed up, had a volvo 280 drive, installing a bravo 3 drive. Can anyone recommend an x dimension for this installation?

I'm pretty damn sure I've already told you what I did and I'm most surely the only person in the world running a Mag 25 / B3 combo.

My 25 Magnum with a B1 was "X" plus 1-5/8"
Probably go "X" plus 1

I'm pretty certain that I've already talked with Gerard about what I did .
Had I set up for a single prop I probably would have gone a bit more up like you did. Funny .. I ended up settling on one and an eighth inches .

No adverse effects and the unit isn't dragging either.
I don't think that the B3's can take as much + trim as the B1's before the props ventilate so I was glad I didn't go up too high and loose the extra leveraging of the + trim .