No worries. Although this is not an approved method, you can use a measuring cup and a bowl to get a rough idea where you're at. Take 1 cup of pool water and add it to the bowl. Use the same measuring cup and water filled to the same 1 cup line, and add a cup of tap water four times. Mix it up and do the CYA test. Multiply your result by 5. Or make it 9 cups of tap water and multiply by 10. The more careful you are with the measuring, the better the ballpark figure, but it's still just ballpark.
CYA gets slowly depleted from pool water by splashout, backwashing, leaks, and also some depletion from oxidation, perhaps 5 or 10 ppm per month if you've been dumping a ton of chlorine in to keep it from turning green. So you won't have every bit of it from the beginning. In really bad pools in your situation, we often see CYA over 300, sometimes 350 ppm CYA, but let's say 400 ppm CYA just as an example.
Assuming a 400 ppm CYA start, 4 drain/refills of 35% would reduce the concentration to 70 ppm CYA which becomes manageable. I haven't heard of 1000 ppm CYA, but if it was, then it would be 4 drain/refills of around 50%.
Nothing wrong with what you're doing, just threw out the layered exchange method as an option to reduce the time, effort and water. Don't let the plaster get dried by the sun. Good luck with it.