I would say that a good rule of thumb is to have your coconut oil and salt percentages roughly equal to each other. So if you only have 25% of your oils as coconut, you can't have more than 25% of the oil weight as salt. Salt NEEDS coconut or babasu oil, or no lather.
Thus the popularity of an 80% coconut oil / 5% castor / 15% other oils recipe with 80% salt & 20% superfat.
If you're having problems with using percentages in the calculators, re-read some of the tutorials stickied at the top of the beginners forums. You can define a whole recipe by percentages, once you enter the total weight of oils and define a super fat amount.