I am now working on a font family. I have two masters - Light and Ultra Bold and I will make 10 more instances to have 12 fonts in my family . I made same spacing for both of my masters (Light and Ultra Bold have “=” in RSB and LSB). While in Light the spacing looks great, in Bold it doesn’t look nice at all. I can adjust spacing only in Bold to make this very master looks nice, but I don’t understand how different spacing in my initial masters will affect other instances in between these two masters. How the Glyphs app count the proper LSB and RSB in other instances? Is it going to proportionally interpolate in between two values of RSB and LFB for each instance?
yes all SBs are interpolated as well and should look fine. However, since a very light font, intended for bigger sizes, is happy with tight spacing, and an UltraBold font also needs tight spacing, a regular interpolated weight will probably be too tightly spaced for small sizes. Then you might want to give the regular instance bit more spacing, either by inserting an extra master, or by adding a custom parameter to the instance definition to globally add some spacing.
Oops, that global parameter is not yet available.
Is the Filter - Transformations parameter not doing it for your use case? I use this to change side bearings of tabular figures to make their widths match between style linked instances (I think the adjustments can be in relative units only, not percentage).