I've had several guys tell me that for PIC they have simply taken 90% of the MP time listed on their AFORMS printout. Basically you know roughly how frequently you have the A code when going out to fly so you're pretty much on your honor to use that number. Probably the best bet is to try to get enough hours so that even if they thought you were off in your calculations it still would keep you above the mins....just my 2 cents