If at all possible, try to get a reading that you're confident in BEFORE you get into the back of the ambulance. Definitely make sure you get a good reading before the truck starts moving because if you're having trouble now, there's no way you'll be able to hear or feel it once you start moving.
Owning an expensive stethoscope also helps more than people would like to admit. Beyond that, practice and don't get discouraged. And don't ever make up numbers - sometimes it is not even physically possible to get a pressure this way due to peripheral artery disease or whatever. Feels bad man to not be able to tell the hospital what the pressure was, but you can use other proxies for perfusion (skin color, capillary refill, mental status, etc).