"...how can you calculate how much SF in % the CM is going to contribute?..."
You don't have to calculate this, because it's not something you can control. Assuming you're doing a CP soap, the superfat will be made up of all of the fats in your soap recipe -- not just the fat from the coconut milk. If you want to control the specific fat that is your superfat, you need to hot process the soap and add that fat after the soap is done.
In the example I've been using, the fat in the recipe is coming from two different places -- the 100% pure coconut oil you scoop out of a can and the coconut fat in your coconut milk.
Figure up the amount of fat that comes from both places. In the first case, it's easy -- 1000 g of pure coconut oil out of the can is 1000 g.
In the second case, the coconut oil is only a part of the coconut milk, so there's no way around reading the nutrition label and doing a bit of math as I explained earlier.
Once you have the fat from the CM, figure up the total fats in the recipe. If I was using 1000 g of pure coconut oil in the base recipe AND adding 45 g more coconut oil from the coconut milk, the total fat in my recipe is 1000 + 45 = 1045 g.
1045 g of coconut oil is the number I would plug into my
lye calculator. I would enter 20% for the superfat and I would calculate the lye using this basis.
The other alternative is to just guess at how much less superfat I should use to compensate for the unknown amount of fat I'm adding with my coconut milk. Maybe the percentage should be 13%? Maybe it should be 8%? Maybe only 19%? It's hard to say without doing the calculation. Since I know coconut milk products vary widely in their fat content, that makes me even less confident that I can make a reasonable guess about how much less superfat to use.
If a person cannot do the math, then the "I'll just guess" approach is an option to consider, but I'm uncomfortable doing that for myself. I'm not saying that being precise about the ingredients I use in my soap is a sure-fire solution for soaping troubles, but I do believe that it may help reduce unpleasant surprises.
PS: the "warts" might be bubbles of soap that raised up from the center of your soap loaf -- mini volcanoes, so to speak. Or actually more like Yellowstone Park "paint pot" bubbles. Pic source:
http://joelachandthekeys.com/photos.html