Unless you're producing for the commercial sector Harry - don't worry overly about it.
Fat = Flavour
But too much isn't good for the pig or your pocket (either in terms of overfeeding or paying butchery charges based on weight)
HTH
Karen
Edit: Oh, it turns out you can measure backfat on a living animal (my mistake) you need an electronic ultrasound device type thingy (and training in how to use it
) But for someone producing for themselves it's probably not financially viable.