Back in ancient times, I used the FontLab's* blend tool to create a weight range. I'd start with one blend, usually 50%, which I'd clean up before making intermediate blends. The blend tool left a mess of crazy-ass beziers to clean up but it gave me a chance to fine tune the medium† weight. I could diminish some of the optical tricks, fix the balance on my diagonals, fatten up the punctuation and get my medium weight looking just right. After that, I'd blend the intermediate weights without much visual adjustment; just cleaning bezier disasters. When I was designing the heavy weight, I could do all the optical tricks I felt like without a care in the damn world because I knew I could fix the medium weight later.
For the last few years, I've been multiple mastering but lately I found that I want to start toying with that medium weight again. And I'd like to go a little crazier with the heavy weight without ruining the medium weight.
What kind of workflow can I use? Do I add the medium weight as another axis? Whaddoido?
* I use Windows so I can't use a lot of the fancy interpolation tools available on other platforms
† not necessarily medium but something around the middle