At about the same time that I did my dressmaking course I also did a pattern drafting course. It focussed on taking some basic pattern blocks, drafting to fit your size and how to make changes to the block for different styles.
What I know about pattern grading is that which appears to be self explanatory from a multi-sized pattern: a size 10 is not just an allover reduction of a size 12 (although I guess for some very simple patterns pieces it may be, like pockets). And the 'everything-must-be-more-complicated-than-it-appears' voice in my head says that the interval between sizes would probably not be constant but rather proportional to changes in frame that accompany changes in size. And I'm sure that there is some arcane pattern drafting knowledge about these things (can I find it at the library?) and expert pattern drafters who know all of these things.
Having said all of that, I graded the size 14 bodice pattern down to a size 12 by just following the intervals and putting lines in what looked like the right places. Why did I have to do this? because the size range in my packet was 14-16-18 and there is no 8-10-12 on the market at the moment (besides which, I don't want to pay for it again).
* A-ha, thanks wikipedia; yes, there's an algorithm for it. *