There's a discussion about tacho calibration in another thread. But if their tachos are calibrated to max rated RPM, and you cruise at 0.8 of max rated RPM (let's say 2160 rpm cruise/2700 rpm max), then you will indeed accumulate 0.8 tacho hours for each hour of flight time. Plus, of course, the taxiing, runup and everything can be written in your logbook as flight time, but are done at very low RPMs. So getting 0.8 tacho hours to pay, for each 1.0 flight hour in your logbook doesn't sound entirely unreasonable for me.
For hour builders, it's the flight hours that count, not the tacho hours.