A very quick little mixing formula is 2.75 pounds of plaster added to 2 pounds of water makes 81 cubic inches.
Here is the finished bat. 50 pounds of plaster and 36.6 pounds of water.
I always line the bat with a double layer of cotton sheet and then pour in the slip. After a couple of days the sheet can be removed with the clay and the bat can be allowed to dry. With these new molds that might not be necessary, but I will probably still do it.