It is all just a guesstimate. You could cross a pure sativa with a pure indica and end up with a plant that exhibited 95% of the sativa traits and only 5% of the indica. None of the breeders really know what exact percent of the retained genes are from which source without just smelling, tasting, smoking, and guessing based on this.
If you wanted you could try and base the numbers on just simple %'s that doesn't mean much. start with a pure indica and a pure sativa and result is a 50/50. Cross that with a pure indica and you'll have a 75/25. Cross that with the 50/50 and you'll have a 5:3 ratio which is a 62.5/37.5.... These numbers wouldn't mean much in my opinion.