What I normally do is climb to about 8,000m, accelerate, climb to 12,000m, accelerate to m2.05, then pitch for a 45* climb. You can get it up to about 25,000m or so if I remember correctly. Don't worry about fuel, you don't burn anything really at those altitudes, and your engine will flame out anyway because of the altitude.
The F-4 that climbed to 95,000ft or whatever accelerated to max speed at 25,000ft, then pitched for 45* so that's what I based my trial on.
EDIT: Re-tried this method to see if I remembered correct or not. It goes pretty high. 88,662ft (27,024m)
This is for a fun, max zoom-climb though. I'm not sure the actual max cruising altitudes, but the best altitude for maximum speed is about 12,350m according to my experiences.