I think there is no one site nor forum that concludes genetic mixes. You would have to read the basic colors, understand which is likely to be more dominant (theories aren't always true), etc, etc. It's too complicated, partly (if not mainly) because bettas have been crossed bred too often thus there is practically no such thing as "basic" genetic. The best way to calculate outcome is to know the genetic background of your breeders - preferably 3 previous generations.
I suggest you read all you can find on genetics, like I've been doing, and do your best to make sense out of all those gibberish
. You can also checkout spawn logs to see results from mixes.
Once you've read the basics, I'll be more than happy to discuss it with you ............ I'm sure others will too.
Here are some you can start on: