Has anyone got extensive experience using a P4 out there, Ive been using one for a while and trying to validate readings, going by Fishers manual doesnt add up most likely because of my location, the fish is tuned and calibrated for certain, ive tested it on land and ts good, in water running over an 18th C iron anchor 13 feet long 30 feet away at most I get 150 - 350 Gamma change, sound right?