Secondly, when you go in for a mix the various attribute of the different flours; make up for the deficiencies of single flour, in terms of taste, nutritive value, gumminess or the binding abilities. For instance, when you use only rice flour to bake, the end product is hard/ gritty. Sorghum flour alone is bland.
The third factor is availability.
The good news is somebody has already done the home work for you and there is the Bette Hagman’s GF flour mix.
2 cups rice flour
2/3 cups potato starch flour
1/3 cup tapioca flour
for a larger batch of 9 cups use
6 cups rice flour
2 cups potato starch flour
1 cup tapioca flour
As potato starch flour was a problem, I have substituted it with arrow root starch, in the same proportion. In India arrow root starch is used for purposes such as thickener for soups, more so in the hotels. From what I have read, generally corn and potato starch are interchangeable. I tried out apple cake with this GF flour mix. It came out fine, but the structure called for more attention and maybe a biding agent as unflavored gelatin may help.