Somewhere I read that after weighing the finished piece, the weight should be divided by .9 for PMC3, .8 for PMC+ and .7 for PMC standard. The result of that division would give you the actual weight of the clay before shrinkage. Now my non-mathematical mind didn't understand how that worked but I did it, for the past two years. Well thanks to info from my friend Carol (check out her site at http://convergentseries.wordpress.com) that formula applies to Art Clay Silver and not PMC products. (Hey my brain doesn't remember the details, just the generalities.) So I guess I've been adding weight to all my pieces.
But wait, it gets better. I may have been adding weight, but my scales aren't accurate (so I've probably not actually been figuring enough weight in the long run.) It's not that they're not accurate (okay, another screw up on my part.) But it has been a long time since I checked them and calibrated them. (Thanks again to Carol for noticing this.) So, now they are calibrated. But I only had a 200 gram weight and my scale calibrates at 100 grams. So now I divide by two after each measurement. (At least that is easier than dividing by decimals for us non math people. Don't tell my other math friend Debbie.)