I had this issue. This is what I did. I set up a siphon (pump would work, too). I got a five gallon bucket and determined how much water in the bucket was actually five gallons. I did that by pouring into the bucket 5 gallons of water from a known receptical (a one gallon container I knew to be one gallon). There are many ways to do this. Once I had five actual gallons in the bucket, I market that level with a Sharpie.
I put the bucket at the end of the siphon hose, and put the siphon hose into the pool, weighted such that that end would stay put. I then started the siphon. Once it got going I put the other end of the hose into the bucket and started timing. Once at the five gallon line in the bucket I stopped the timer, dumped out the bucket and repeat that several times. This gave the rate of the siphon (the average of each bucket filling).
I then reset the timer (stopwatch mode) and let the siphon run. Some quick math gave me an approximation of how long to run the siphon to remove X amount of gallons.
That's all that was needed to remove the amount of gallons I needed to, but then I took it a step farther (of course I did!). While the pool was draining, I regularly measured the level of the surface relative to a fixed object on the deck. I used a straight edge laying on the deck that overhung the pool. Basically, I was timing how long it took the pool to drop exactly 1". I recorded the amount of time that took. Then I did the same for the next inch, and the next. Now I had an average of how long it took to siphon 1". Since I had previously determined how much water (in gallons) was leaving the pool per minute (from the siphon calculation), it was a little more simple math to determine how much my pool held in an inch of water.
It turns out it was very close to 200 gallons per inch. Because I repeated and averaged everything, I feel it's pretty close (and close enough for the purpose of a Pool Math water exchange calculation).
So the next time I need to exchange water, I don't have to repeat all that timing and measuring, and I don't have to use the same exact siphon hose, I can extract the water any way that is convenient, fast or slow doesn't matter. I know my pool is 200 gallons per inch. If I need to get rid of 1000 gallons, I have to drop my pool level by 5 inches! EZPZ.
Your pool's shape can still effect even that. Stairs or benches can affect the measurement, as can the taper of the sides. For example, as the pool tapers from it's widest point to a narrower shape near the bottom, the gallons per inch measurement would change. But nearer the top, there the sides of the pool are relatively vertical, the measurement will be fine. I can drop about 24 inches of water, or more, using my gallon per inch calculation and be very close.
If you put in a little extra effort this first time around, with the bucket and the tape measuring, you'll get your own gallons per inch measurement that you'll then be able to use for water exchanges, for the life of the pool.
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