Hey man. Im BioSig certified. You cant guess, the readings have to be very precise. For example, 3 mm on the tricep is optimal, 6 mm, not so good. If you've done caliper testing before, you know that a 3mm variation can happen pretty easily.
Without knowing the norms, you're just guessing whether its a good value or not. Also, the measurement has to be taken at an exact location.