EDTA should be added to the water phase of the formula and as early as possible in the formulation process. By adding it early on, you make use of EDTA’s benefits to your formula. It is recommended to be used at 0.1%, 0.2%, or 0.3% in soaps, but that depends on the purity/strength of the EDTA you are supplied with. A lower purity EDTA supply might require you to use more of it in the formula.
Here is the link where I recovered the above information, (Dow Chemical):
If you are going through a home crafter website, ask them who supplies their EDTA ingredient and ask them for the formulating specs. The specs will tell you the proper percentages to be used, based on the manufacturers recommendations.