Think about cubing strains...
100% Sativa x 100% Indica = 50/50
50/50 Hybrid x 100% Indica = 25/75
Basically you water down the sativa or indica genetics using another strain. If you cross a clone-only female to a male, then cross it back to itself by reversing the chosen offspring and pollinating the clone, you essentially get some % of the parent (because it's represented twice in the lineage) and some percentage of the breeding offspring (because it was crossed back to the parent). This is how you make autos, essentially...find a great strain (WW for example), cross it to an auto, back cross selected F1 to the parent, find a fast/auto pheno and then line breed to that trait, possibly back crossing to the P1 parent again or back to another auto if necessary. That's where the magic comes in...that's also why I hate all these polyhybrids dumped on the market these days lazy people just throw feminized pollen around to other name-brand strains for name recognition.