Go to the post you wish to delete and click "edit"....you can delete it there.

Math says your CYA should've dropped more....I have no explanation unless you drained a little less than half.

80 CYA is manageable if you'll keep your Cl level around 5-10ppm constantly. Add enough muriatic acid to the pool to get down to about 7.2-7.4. The Alk will come down with it and shouldn't require any adjustment.