It is very simple when you it is represented in Punnett's square. Here is a good article on it
The Periodic Table of Pythons - Genetics 101
I also wish to point out that when gene combinations are expressed as percentages, and are often seen in the law of averages. But sometimes it doesnt always work to those percentages either. A possible het means only possible, and as far as I am aware the only way to test the het is to prove it out in breeding. I am yet to meet a breeder who can visually tell the difference between het and non het in a clutch by looking at them, it is only a guess based on percentages, or on proven genes. My opinion is that good breeders sell possibles as normal, and only known hets as hets. Remember in theory any snake can be a het. I am not an expert so please dont take my opinion as gospel, it is just an opinion.