You can use co at 100% and SF at 20%, no salt, and it's a soap that many people enjoy. It's the superfat which is key to high co recipes.
When using embeds, you are essentially combining the two recipes at a certain ratio, watering one down with the other, so to speak. If you have a 100% lard bar and you add in embeds of Castile at a ratio of 2:1 lard to embeds, you have on average a soap that is 1/3 olive and 2/3 lard, depending on the distribution of the embeds.
Eta- if you're adding embeds then you can just change the recipe to make it less drying. No need for embeds