Dave,
I think a thinner water level works best, adding a thickener would require a thicker water level.

My idea wouldn't be a more complex setup, just a typical light box and a simple water thickness checker (I'll make one up to prove it out). I could write a simple Excel program to calculate the fringe pattern given the viewing distance and water thickness. This would keep water as the reference instead of your smaller flat. It would be nice if this could be incorporated into a fringe analysis program.