Hi Dan,
I agree with the two previous responses. The design of every individual dish and LNBF support arm (without the back-mounting assembly) could have its own specific "built-in" angle. This would require the mounting assembly / elevation bracket to be marked or calibrated to compensate for that angle. So, if you swap out the front end of any dish assembly with that of another, it's just a gamble on whether the elevation scale matches or not. I would suspect that most would be within +/- 5°, though.
If you are starting with a dish that is already in service and working well, I think you can start with the same elevation angle. Pick a known strong TP signal and then just wing it. If you keep your azimuth alignment the same (i.e. change the reflector w/o taking the assembly completely apart or taking it off the mast or at least marking the mast so you get it set back in the same position), you should realize that you only have to go up or down in elevation to seek the original signal and optimize it.
RADAR