Does the employment agency have an on-line calculator that I can use to determine the unemployment benefit amount for the amount of pay that an employee was receiving.

One is unemployment-benefits.org, but also various states have them, since the amount varies by state.
The way you word your question is unclear though. It sounds like you want to find out what someone earned for wages by working backward from a known unemployment check. That won't be possible, even if you keep trying different numbers to arrive at it, because it isn't linear.