Hmm, a tricky one here. It all depends how well you keep your food inside
I can have a large breakfast and go straight into the gym no probs, but not everyone can. I would recommend scale it up. Have a 2 block BF, if all goes well the next day have a 3 Block BF, and if all is good there take it up to 4. With the lower blocks take a snack for immediately after the Work Out to complete your 4 blocks, so your muscles have enough food to feed from.
As you progress you might have to up the fat blocks some more for even more energy. I've heard there are CrossFitters who take X5 times the amount of fat, but I think those are the ones on a different planet than us mere mortals