BTW, the circular slide rule is no longer mandatory in the IR or ATPL exams. I don't know if it is mandatory in the CPL exams.
It wasn't needed on mine JAA CPL (or IR). There was a question that needed IAS(CAS)->TAS conversion but "the rule of thumb" TAS=FL/2+CAS was accurate enough (to be closest to the right answer, as you do on these tests).