More handy width/side-bearings customization

[ghost]

Is your feature request related to a problem? Please describe.
For now in custom builds there is no way to automatically build regular and extended width variants of font with the same glyph sizes because side bearing parameter is not a proportion but an exact value. So every build you have to manually adjust parameters by yourself.

Describe the solution you'd like
Adding proportional side-bearing parameter while keeping standard one. Proportional value should override standard if set.

Describe alternatives you've considered
Ability to set side-bearing value for each specified width separately. Moving sb parameter from metric to width block.

Additional context
By the way can anyone explain me how the emu unit works? Especially with sb parameter. I thought I will be able to get the same glyph size on extended 600em width by multiplying sb with 1.2 but it didn't worked and so I have to set the value approximately.

[be5invis]

There's already a mechanism to use a scale in metric-override (as well as using an offset):

[buildPlans.iosevka-custom.metric-override.multiplies]
sb = 1.0625

[buildPlans.iosevka-custom.metric-override.adds]
sb = 15

[ghost]

@be5invis and again it just works thank you for your patience. But I still wonder why manual value multiplication didn't work the same way?

[be5invis]

Let me introduce expression-style metric override. Now, you can do this:

[buildPlans.iosevka-custom.metric-override]
sb = 'blend(width, [500, 60], [600, 110])'

This configuration will set sidebearing to 60 when width is at 500, and to 110 when width is at 600.

Closing as fixed.