Hi Stacey- when I use EDTA, I use .5% as per the total weight of my batch, i.e., oils, water, lye, colorants, FO and whatever other additives I choose to use.
If you are using the EDTA in powdered form (as opposed to the 39% solution form), the easiest way to calculate is to just multiply your total batch weight in grams by .5% in order to get the gram weight of the EDTA powder to use. Note: when adding the EDTA to your batch- powdered EDTA dissolves best in warm/hot water.
If you are using a 39% solution of the EDTA (which is the form that I use), a little more math is involved to get the correct amount to use, but it's very easy math. Just add up the total weight of your batch in grams and multiply by .5% as I mentioned above, then take the resulting sum of that and multiply it by 2.56. The resulting sum is the amount of 30% EDTA solution (in grams) to weigh out for your batch.
Example- if the total weight of your batch is 6.5 lbs/2948g and you are using the powdered form......2948 x .5% = 15g powdered EDTA (rounded up) to add to your batch.
Or, if you are using the 39% liquid form of EDTA...... 2948 x .5% = 15g........ 15g x 2.56 = 38g EDTA solution to add to your batch.
HTH!
IrishLass