Hi, that’s of course annoying if you purchased the RMX tools and are now stuck with an expensive piece of software that doesn’t work for you. Yet, it may provide some incentive to flesh out your design to incorporate multiple weights.
The reason RMX requires multiple weights is the following idea: in order to calculate e.g. an regular synthetic small cap H, you take a bold H and a thin H. If you just scaled down a regular H, the result would have much thinner strokes vertically and horizontally, of course.
So what if you interpolate a much bolder H and then scale it down? You have to interpolate an H to be so bold that if you scale it down to, let’s say, 80%, your stroke weights end up the same as the original ones. So you have to interpolate the stems to be 100/80 as thick as your original H, then scale it down to 80%. You see, already for this to work you need two weight masters.
Now, bonus: what if you want to scale it to be 80% in height but 90% in width? Small caps are usually a bit wider in relation to their height compared with their respective caps. This means you utilise anisotropic (unproportional) interpolation. For the above example, you interpolate (100/80×100)% on the y axis and (100/90×100)% in x direction. Then, if you scale it down to 80% in height and 90% in width, your strokes will end up the same thickness as when you started.
Sorry for the lengthy explanation, but I remember when I first learnt about this concept and applied it in my design workflow. If you want to do it the really old school way, you can use FontLab, using the Blend function between two fonts. You’ll get your hands dirty and familiar with the mathematics behind it. Enjoy!
I’ll leave you with Tim Ahrens’ article which he wrote on the topic, it’s really fascinating and explains the concept much better than I have been able to.