There is a range of acceptable dosage. You want to use enough to make your soap work better in hard water, but too much of a good thing is not necessarily better.
For citric acid, the dosage is anywhere from 0.1% (1 gram per 1000 grams of oil) to 3% (30 grams per 1000 grams of oil). I think most people use the 1% rate, to be honest.
And, just to be clear for all reading this, you'll also want to add to your soap recipe an extra 6 g of NaOH for every 10 g of citric acid added. This extra NaOH is needed to convert the citric acid to sodium citrate. If you don't do this, the citric acid will "steal" the lye it wants from your soaping lye and that theft will increase your superfat.
Because the molecular weight of sodium citrate is higher than citric acid, the dosage is a wee bit higher, if you want to be super precise -- 0.13% to 3.9%.
You do NOT, as Irish Lass pointed out, need to add any extra lye when using sodium citrate.
