Blender's UI system has it quirks, I don't know if there is a way to have essentially the leftmost column have a fixed pixel width and the rightmost extend infinitely. You don't want percentage with split because in your example it would be set to something like 12,5% but if you use the panel in its regular size you wouldn't be able to read the text in the left column.
But this does beg the question, what is the reason you need to have a panel this wide ? It could be due to the BlenderBIM addon's UI which does not always bode well with the general design of Blender. Much information to display, but not much space to display it. Also in Blender lists almost always contain vertical information, not horizontal.
Comments
Would this be the way to do it?
https://blenderartists.org/t/keeping-a-uilayout-box-a-specific-width-percentage/531111/3
Blender's UI system has it quirks, I don't know if there is a way to have essentially the leftmost column have a fixed pixel width and the rightmost extend infinitely. You don't want percentage with split because in your example it would be set to something like 12,5% but if you use the panel in its regular size you wouldn't be able to read the text in the left column.
But this does beg the question, what is the reason you need to have a panel this wide ? It could be due to the BlenderBIM addon's UI which does not always bode well with the general design of Blender. Much information to display, but not much space to display it. Also in Blender lists almost always contain vertical information, not horizontal.
If the right column 'wrapped' text, it would probably not be as crucial to make the right column wider to see everything...and maybe even preferred.