First, please use weight instead of using volumetric measurements. A milliliter of extract or terps does not always equal a gram, and especially when doing larger batches, those small differences can add up to big differences between what was intended and what is actually produced.
Second, potency (of terps in this case) is a simple calculation. Total weight of added ingredient divided by total weight of the formulation. If you added 100g of terps to 1000g of distillate, the total weight is 1100g. 100/1100 = 8.59% terpene content. You are correct in your belief that you would add 900g of distillate to 100g terps to get a formulation that is 10% terpene content.
@GoldCoastTerpenes is on here. Maybe they can explain why either their (simple) math is wrong or how we understand potency incorrectly. I regularly formulate with this basic equation, and I pretty well nail terpene potency every time. I’d really like to hear how and why our math is incorrect.
If you crc your material with 1.5% b80, do you do starting material x .985=y, starting material-y=grams of b80. Or do you do (starting material x 1.015)-starting material?