3/4 inch sounds about right especially if raised outdoors.
The slaughter house will record the dead weight/ fat probe and print it on a tag which goes with the pig to the butcher so you could just ask the butcher when he gets the pigs in, or just ask him to save the slaughter tag. We sell direct to a local butcher so I phone the slaughter house at lunch time (having dropped them off in the morning) to get the weights for invoicing purposes and they just give me the fat probe as a matter of course.