I have a recipe that I'd like to make that calls for 3oz of 72% cacao chocolate. I have plenty of 60% cacao chocolate, and I have Hershey's cocoa powder. The recipe is just for brownies, so absolute precision and supreme quality isn't required, but I would like to get the right level of "chocolatyness". 3oz = 85g. Going from 72% cocao to 60% cacao would leave me with a deficit of just over 10g of cacao. If cocoa is basically 100% cacao (is it?), then it would seem that just adding 10g of cocoa would be close enough to "right".
Since I'm dealing with chocolate, not anthrax, I'm disinclined to worry about the 10g (or whatever) surplus total chocolate weight.