kb2112 The ideal thing to use would be actual pork fat from a butcher but that can be hard to find, untrimmed pork butt however is available in most grocery stores or butcher shops. 20% fat content for an untrimmed pork butt would be pretty lean, we find they are generally closer to 30% and some have larger fat caps that can increase that even further.
However, your math is correct, if you are looking for a true 50/50 split you will end up with somewhere around 15% fat which is a little leaner than is ideal. My first suggestion would be to try to get some pork fat from a butcher and add that to get your fat up to about 20-25% range, which is what we think is a good range for a juicy product. If you cannot find any fat then yes, add more pork butt than to get a better ratio.
Let me know if you need anything else!