Not that this is a way to measure the water your draining, but if you put a Tee in the drain line from the overflow, and 2 valves after it, one on the return, and one on the out of the Tee, you could shut off return pump, close vale on the return line, open valve on tee, and drain all the water that will siphon down the drain. Should be a pretty consistent amount, but will it be enouh for your water change. I have mine set up this way on my 90, and with my plumbing I get about 10 gallons.