Before you start draining....
Mix 2 parts tap water and 1 part pool water. Use this as your water sample and multiply the result by three. Or use 3 parts tap water and 1 part pool water and multiply the result by four. Just be VERY precise with your water measurements as doing a diluted test inherently adds more possibilities for error. The more you dilute the pool water, the less accurate the test becomes.
If you were to drain 1/2 now and your starting CYA was 300, you would still have a CYA of 150 when done refilling. And you just added 10,000 gallons of water to get to only 150. Then you'd need to drain 1/2 again and refill to get to 75-ish. That's 20,000 gallons of refill water. If your CYA was 300 and you drain 3/4 and refill, that would only be 15,000 gallons of refill water to get to CYA 75. Don't waste time draining until you have a better idea of what your actual CYA is.
As for a pump for draining, many rent from from Home Depot or similar. You can also buy one, but those don't pump a lot of water per minute or are expensive. Last I checked, HD was around $50 to rent a sump pump to empty your pool.
I am doing cya test under cover as part of our pool deck is under a three season room. So it’s a little bit dark. Might try on the other side of the pool where there is no cover, as I just read lighting matters.
You should perform the CYA test in bright sun, your back to the sun, CYA tube held at your waist. Fill to level (100, 90, 80, etc.) then hold at your waist, and glance. If you see the dot, fill to the next level and repeat. If you don’t see the dot, your number is the last level where you did see the dot clearly. You can pour the solution back into the bottle and try again to be sure.
The CYA test maxes out at 100ppm. Your pool store's tester probably has 99 as the upper limit if it is an electronic tester. As you have seen, the CYA test is the most subjective of all the tests run on pools and it has a margin of error of +/- 10ppm. When it looks like a member's pool could be higher than 100ppm we recommend a dilution test to get a ball park estimate of what the actual CYA level is.