OK, here's how I'm going to stop worrying and learn to love the bomb: this PDF is a simple chart I calculated in Excel for a 300mm, based on the formula Ian C passed.
There are two important thing to remember: at 300mm we have infinity focusing, so no bellows factor. At 600mm we are at 1:1 reproduction ratio, which needs 2 stops compensation. Beyond that, it's all tight macro work.
Which means that for any non-macro work with a 300mm, I should expect at most a 2 stops correction. Negatives being negatives, minute errors will not be dramatic. Still, what I did in my chart is to put in bold every 1/3 of a stop increases.
So, armed with a measuring tape (or the camera's, since it's a monorail), I do a quick check of bellows length, match with my chart, and I can adjust exposure within 1/3 of a stop.
The next time, with a different lens, I just have to recompute my table.