PAR Readings

I have two of the Chinese LED fixtures. MARS Aqua dimmable lights. I really would like to know how much PAR they are giving my corals at top, middle and bottom of tank but cannot see spending money on buying a PAR meter. Called a couple of LFS and no one would rent them.
How do you guys figure out at what strength (10-100%) to put your lights without measuring? Right now I am just guessing and have them at 70%