You’ve hit the nail on the head here. This is very much where the limitations lie, especially if you’re interested in text scaling and there just isn’t much MainUI can do about it.
There are workarounds out there for some of these problems. The easiest, most of the time, is to continue to use % and just do the math (e.g., B is 50% of A and I want C to be 33% of B so I have to use 16.6% since the % refers all the way back up to A). But for more complex things, there might be other solutions. For example, if it’s a more graphical display you might be able to render it as an svg. If it really comes down to it, there are (fairly hacky) ways to get the widget width.