Wayfarer - we aim now for 15 mm back fat, if you achieve that there is a 1/2 to 3/4 inch layer of fat on the fattiest pieces of loin and seems just right; the lowest we have ever got to is 9 mm. Slowly over the years we are getting there, mainly by keeping their regular feed low from 2.5 months old, building up to a max of 1.85 kg/day (= 4 lbs) by 4 months of age until slaughter. I don't think last minute diets work at all, as the fat builds up throughout their life.
We have always found it hugely difficult to judge beforehand just how fat they are going to turn out. All our pigs seemed to us to be quite slim when we sent them off, then you get back a slaughter report with anything between 25 to 35 mm of fat, which can be very off-putting as there seems to be more white than red and you have to trim masses of excess fat off. Probably this level of fat was acceptable a century ago, but everyone we sell to nowadays wants relatively lean meat, including rare breed pork meat. We have always found our saddlebacks produce less fat than the tamworths, fed exactly the same rations over the same time period, but I have no idea about pietrains at all.