Allowing % Scaling of nested Auto Layout Frames

If I have 2 auto layout frames nested inside of another e.g.

Main Wireframe (Auto Layout)
→ Side Bar (Auto Layout)
→ Body (Auto Layout)

I want the side bar to be responsive and always scale as a % of the overall Width.

I’ve been able to do this using variables but only at fixed Widths. Ideally I’d like to resize the main window and the required frame should maintain the 25% or whatever % scale.

It can be done with constraints if I remove auto layout however that kind of goes against the whole point of using the power of auto layout.