When substituting white rice flour for AP flour does one use equal measures?
Depends on what the recipe is, but in general, rice flour alone cannot substitute for wheat flour.
I recommend making a blend of gluten-free flours to sub for all-purpose. I usually use a mixture of rice flour, sorghum flour, millet flour, and cornstarch (you could also use tapioca starch or glutinous rice flour instead of cornstarch). All of these are available from Bob's Red Mill. If the baked good in question needs to hold together (i.e. a quick bread or something like that), I'll add a teaspoon or so of guar gum.
Rice flour & AP have completely different properties, not sure what application you require the substitution for..
I can only echo what everyone else has said. Rice flour does not have the protein that AP Flour has. I would check out this post on Gluten Free Girl and the Chef about making your own gluten-free baking mix (I assume that's what you're using it for): http://glutenfreegirl.com...