That water volume seems very low for this size pool. How deep is your deep and and how deep is the shallow end? For 16500 gal. an 18x36 pool can only be 3.4 ft deep.

If you dropped the level by a ~ foot (11") then that is 18*36*7.5 = 4860 gal or 29% of the 16500 total in a flat bottom pool. So your ppm of cya is also dropped by 29%.

But I question the veracity because it's based on assumptions without enough information.

You can just let the CYA drift down on its own via the occasional refill. Do you have a sand filter? Backwashing and rinsing uses up water that has to be replaced. Use more chlorine for now, let time pass, test, and adjust your chlorine additions accordingly. All a high CYA really means is that you need more chlorine, as you've found out.

In the meantime, you might walk around with a measuring tape, record the dimensions of your pool (L x W x D) and make a note of its optimum volume. That's really useful information to have.