I'm not doing it in a 'commercial' sense - I don't sell my pigs to the abbatoir, I prefer to have the carcass butchered and sell direct to the consumer.
I keep traditional native breeds (which are far more prone to high fat levels than a commercial breed such as Landrace, pietrain, Large white, hampshire etc) I'm not weighing on a weekly basis, trying to get to finishing at X weeks old - I'm letting them grow slowly, in large paddocks and they generally get a compound feed with around 16% protien as well as lots of fruit & veg. My backfat measurements are generally around 15mm (which probably seems really high to you, but for rare breed pork I don't think it's actually that bad)
When I say I feed 'by eye' I mean that they generally get around a pound of food per month of age, but if they start to look or feel skinny (obvious backbone, condition score of less than 2.5) I up it a bit (or reduce the size of paddock they're in) and if they start to get fat (can't feel ribs, condition score of 3.5 or above) I cut back the feeding or move them to a larger area.
It's not as exacting as the commercial guys who worry about feed conversion rates and have to produce a carcass for £x for the butcher/supermarket........I'm a bit more laid back about it. But then, I'm not producing huge numbers of pigs either
Hope that explains it a bit ?
Karen