You CAN calculate the active chlorine level from the FC, CYA, pH and temperature using the PoolEquations spreadsheet, though for warmer temperatures you'd need to set line 230 "Use Temp. Dependent Cl-CYA" to TRUE in columns B and C.
At a pH of 7.5, a rough rule-of-thumb is that the active chlorine (hypochlorous acid) level is half of the FC/CYA ratio.