You can't really do that though. I mean at a high level, yeah a poor diet and little exercise will almost certainly lead to an adult gaining weight. But it sounds like you're describing the possibility of calculating his expected weight gain based on how much he ate and how much he exercised, and I don't think that's possible. Not to mention that it's been widely reported (iirc even from Spurlock himself) that he was drinking alcohol heavily during this period, which will further complicate this calorie-level reconcilation you're suggesting.