Just consider how many iu/shot you want and what is an easy volume per shot for you to work with. If you want a 15iu vial to be the same strength as a 10iu vial with 1ml of water, just add 1.5ml of water to the 15iu vial. That will give 1iu per 1/10ml, so you would inject the same volume per shot as you would have in the past if you're going to be dosing the same as you did before.
I'm assuming the wafer adds a negligible volume but I don't know for sure since my gh doesn't require reconstitution. If you use, for example, 15 shots at 1/10ml per shot but have a bit left over, you'll know I was wrong about the wafer adding volume.

I doubt it would tho.