"I guesstimate it is between 360 and 480 gallons. It has an opening of about 5.5' x 5', it has 4 deep seats - the deepest part of the spa is 23.5" deep"
I question if you are adding chemicals based on the wrong gallon-age. If you spa is 5.5' and only 23.5" deep I don't think you have 360 Gallons. Mine is 7'X7'X34" and it only holds 320 Gallons. Your manual should show you exact capacity.