The plaster in the bottom is the thickest and is trolled to be straight. The plasterer is not going to use a transit to see if it is level, it's just going to be what it is. After working a year and seeing the construction of about 160 pools, I've never had a customer call back because the bottom of the pool was not level. I think it is a non-issue. The location of the main drains should be in the deepest part.