Combined chlorine is total minus free.

Yes, that’s the standard method.

Most DPD test kits measure free chlorine first. Then you add a reagent to measure total chlorine. Subtract the free number from the total number. The difference is combined chlorine (aka chloramines — the stuff that smells like a locker room and burns your eyes).

You want combined chlorine below 0.5 ppm. Higher than that means your pool needs a shock.