You can use the same recipe calc for both HP and CP. In either method, the saponification reaction doesn't change.
The typical difference between a recipe for CP and one adapted for HP is most people reduce the lye concentration to 25% for HP (that's the same as 3:1 water:lye ratio). That will not change the alkali weight, but it will increase the amount of water for the batch.
The reason for this -- there is typically more water evaporation during the HP cook, so the soap is easier to make if you start with more water. That said, I've made HP with 33% lye concentration (2:1 water:lye ratio) and it went tolerably well, although I think 25% lye concentration is a little easier.
Some people want to add superfat after the HP cook is done, which is something you can't do with the CP method. I've seen some unusual ways of calculating this "after the cook" superfat that make sense to them, but seem overly complicated to me. I just use the regular calc as usual and simply reserve however much fat I want to reserve for adding after the cook. It might be that this is the type of thing you're encountering, but that's just a guess.
If you want more specific details, provide a recipe that seems "off" to you so we can see what you're talking about.