Paint thinner for soft ablative bottom paint
I wouldn't try to stretch bottom paint by thinning. Unlike other paints who's purpose is to provide a protective coating, CPP bottom paint contains copper which prevents marine growth. Thinning will thus reduce the effectiveness of the bottom paint, possibly resulting in more growth.
Better to buy an extra quart or gallon and save the extra for the next season. It doesn't go bad.
Sabre 38 "Victoria"