just weigh the amount of PB you get in a spoon and calculate it from there. You should be able to find out how much 100g of PB is.
Order of exercises are usually big to small. For example, one wouldn't usually train forearms prior to deadlifts, since training the forearms could seriously hinder performance in the deadlift, leaving the hams, glutes and low back to get less of a workout than they otherwise would.
But what you prioritize also comes into play. For example, if you do benching and squatting the same day, most people would say to do squatting first, but if you really prioritize benching for whatever reason, you could stick that first. (since the beginning of the workout is usually when you have the most energy)