A snake either carries the gene or not. The problem comes when you breed 2 snakes and the offspring of the 2 include both normals and hets.
-> 100% het means that all offspring in the clutch will be hets. This is only possible when one of the parents is homozygous (visual) for the gene in question.
-> 66% het means that there is a 66% chance that the snake is het, this comes from het x het pairings. This doesn't mean that any of the offspring are actually hets though.
-> 50% het means that there is a 50% chance that each of the offspring are het. This comes from het x normal pairings. Once again this doesn't mean any of the offspring are actually hets.
I have heard of many het snakes that take multiple clutches to prove out. If BP breeding was just a matter of calculations things would be much easier.