If you are using the dry CYA, you will need to wait a week after putting it in for it to show up on your test. Best that you put 75% of what you think you need, then a week later test, then adjust.

Once you have the sock in the skimmer, chlorinate as if you are already at your target CYA level.

FAS-DPD test kit (such as the K-2006) is a must for this method of pool care.