The entire range of suggested dosages I've found is what Earlene reported -- 0.05% to 4%. The typical dosage used is around 0.5%. I explain all this in my article.
I personally have tried 1% to 3% and found the extra EDTA is not helpful. EDTA also reacts with the lye so larger amounts will cause the soap batter to have a pudding like texture that can be hard to work with.
I strongly suggest sticking with no more than 0.5% max for soap scum control and rancidity reduction. If a little works as well as more, then stick with the lowest effective dosage.
If your scale is too imprecise to measure this correctly as powder, then make a dilute solution of EDTA so you have bigger weights to measure. That will reduce the margin of error. Or put a new scale in your budget?
