It's a pretty tedious process, but if you really want to be within safe levels, here's how I calculate for EOs:
1. Get the MSDS sheet from the manufacturer of your oil. It will list the main constituents of the oil you have. Constituents vary, so don't rely on what someone else is using or says is safe - get the sheet for the specific oil you're using.
2. Look up the constituents in the IFRA library:
http://www.ifraorg.org/en-us/standards-library
3. Calculate your safe EO amount based on the max. amount for each constituent in your oil blend.
So, for example, the Cassia essential oil I have contains 85% Cinnamic aldehyde - this is what makes it smell like cinnamon. (You can find that info on the MSDS under section 3 -
https://www.newdirectionsaromatics.com/msds/MSDS_CassiaEssentialOil.pdf) When I type Cinnamic aldehyde into the search box on the IFRA site and then click on it, a box pops up with recommended max. for each category of product. Category 9 is soap, so that means I should not use more than .05% Cinnamic aldehyde in my finished soap. In the interest of not making my head explode doing math, I just assume my Cassia oil is 100% Cinnamic aldehyde and use .05%. It doesn't seem like a lot, but I get plenty of cinnamon scent coming through.