Timing a 50 gallon drum fill is certainly the cheapest way to go but accuracy will be about +/- 25%.
Remember that by diverting flow from the filter you have eliminated all the head loss between filter and pool return, including heater, return valves, check valves, and any 90 ells. Typically greater than 1/2 of your head loss is in the return side.
If a really rough estimate is all you need this will get you there.
Calculating head loss:
http://blog.poolproducts.com/swimming-pool-feet-of-head-calculations/