The hammer test is meant to study the uniformity of concrete, not the strength. There is an ASTM about the use of this NDT. Research and producers have provided formulas that relate hammer measurements to compressive strength, but I would not trust them if they aren’t complemented with some destructive tests.
If the structure you are evaluating is old, most likely, the surface will be affected by calcium/carbon reactions, which make the surface very hard. This can cause huge errors in the interpretation of the hammer test.
Each concrete hammer test has several calibrated curves (also called conversion curves) provided by the manufacturer. These conversion curves display the relationship between compressive strength and rebound number. So, you just need to check the manual of the rebound hammer that you are using in your research.
For example, the link below gives detailed information about how to estimate compressive strength of concrete using Proceq-Schmidt Rebound Hammer
There are some misgiving in the use of Schmidt hammer to determine strength of concrete. I think the technique should not be used in isolation but with other techniques as well as core boring. Meanwhile, the correlation curves given by the manufacturer could only be used as guide because you are possibly be using different concrete.