You always want to use the same weight of the original flour when substituting with another gluten free flour. See my Gluten Free Weight Conversions Chart.
However, if you are substituting by the use of measuring cups, the approximate substitution would go as follows:.
Using the above linked chart, 112 g of sorghum flour is equal to 1 cup. So, 75 g sorghum flour = 2/3 cup. And 95g oat flour = 1 cup. So, 3/4 cup would weigh 71 g. You see, they are very close in weight. Therefore, if you wish to substitute sorghum flour for oat flour or vice versa, you can just use equal weights or the above formula for cup measurements.
Thus far, I have only tested this theory in bread. However, I have tested other ingredients using the same weight of ingredients that were used in a traditional recipe and it worked it out extremely well. Of course, you need to begin with a good recipe and the correct ratio of starches, flours, gums, or gum alternative.
Hopefully you can use this tip in your gluten free baking. Let me know how it goes by commenting below.
Find more useful Gluten Free Substitutions.