Susie has a valid point. If you decide to use a preservative, however, the dosage is always based on the total weight of product being preserved. Be sure to use a preservative that can handle the high pH of soap -- I use liquid Germall Plus.
ETA: I read your post more closely. You would still calculate the dosage based on the total weight, even though the base may have its own preservative. The function of a preservative is based on concentration, so if you don't get the concentration high enough, the preservative won't be effective. That said, you do have some flexibility -- I might base the dosage of my preservative using the mid to lower end of the recommended dosage range.
For example (speaking from memory, so don't assume I'm right), I think Liquid Germall Plus has a recommended dosage range of 0.2% to 0.5%. I usually use 0.5%, but if I were adding LGP to something that was contributing its own preservative, I might use 0.3% of LGP or something like that.