2 quick questions about PAR testing

[cvrle1]

Thats a valid point. My thought process was that if I did initial test with empty tank, it would at least give me good starting point. I know that once rocks are added that will change PAR reading around them, but PAR should remain the same above rockwork, no? Tallest rock structure will be around 14" with sand bed, so from top of the rock work to top of the tank (24") should have same reading regardless of rocks. Am I missing something?

Thanks