I personally do not think any 800g Ball should be kept in a 15qt tub.

One of my normal girls is just around 900g and I'm going to put her straight in a 41qt.

As my Spider BP got around 400g I had to switch him out of his 15qt. He simply did not seem to have the correct amount of space. I love giving my Balls some sort of space to be able to stretch out and move around in. I personally like to go with this (includes only a dish):

New hatched - 200g : 6qt
200g-400g : 12-15qt
400g-600g : 28qt
600g-800g : 32-34qt
800g - 1000g+ : 40qt (males), 41qt (females)

Although in my tubs, I always include a hide except for the adults.