I'd use the whole cake unless you want to save some for something else.
Did you make a starter for your 1.054 belgian? My only concern would be the stress placed on the yeast if underpitched on the first batch.
I wouldn't be at all concerned about overpitching on a home brew level. I've used the entire cake of 3787 from a patersbier for a batch of belgian dark strong ale. Turned out great. I would recommend a blow off setup if you do that though. That yeast can be a monster.