It can be uncomfortable (particularly for the over 50's or infants) to descend at greater than 500fpm in an unpressurised aircraft, therefore the ROD is most important to me.
I take airfield elevation, add 1,000 for the circuit, then deduct that from my current cruise altitude and multiply by 2 for my time to start descent ahead of my ETA. Eg cruise at 9,500 feet to a sea level airport, you're looking at losing 8.5 (thousand) feet. Descent should begin 17 mins prior to the eta. If you're going to use increased speed (than planned) for the descent, bung an increment on. In this case, I'd be looking at 20 mins out.