Yep! You have good math.
The X factor is that *some* (not all) recessive genes in the Het form are capable of leaving clues that can be observed by a keeper familiar with that genetic line (aka distorted patterns/lightened coloration/oversaturation of pigment/ringers).
If the snake has zero deviations-then you are stuck relying on genetic odds...which I think would be awful and probably not worth the time/effort to proof breed...