Question about finding right oil mixture for homemade soap?

We are trying to make our own soap to sell, but we only have few months before I go back to school. So we think that we could make a few bars of 100% coconut oil, 100% shea butter, ect.. and then melt them back down and mix different amount together until we come up with a combination that we like. This would be in lieu of a lengthy trial and error period where we try like a 50/50 combo or a 20/80 combo. Right now we are melting some store-bought soap to see if we can prove our theory, but it doesn't seem to be melting or working well. Do we have to just make good guesses at mixtures and wait to see if its a good batch or what? Thanks a lot.