To tell the conclusion first, you can't.
But there is something to be understood about it.
As known, Line Weight can be set only for default scales. But we are using various scales for various views. I've searched lots of threads in lots of Revit communities, but I couldn't find any clear answer. Someone says 'average' or other says 'nearest'..or so. Actually, I've also never thought about this issue seriously before. I'm not that sensitive guy, you know.
So I carried out a quick test to find out what happens in line weights in custom view scale.
I just drawn a line with #5, and measured it's thickness in views with zoomed in.
For example, in a view with 1:300, it follows 1:200. In 1:400, it follows 1:500....
Then, in 1:350? It's at the exact middle of 1:200 and 1:500? The result says it follows smaller one, I mean 1:200.
So, I could make a simple table of Line Weight Setting for Custom Scales as follow.
|Set Scale||Apply Scale|
Above is for the case which has a setting for all default scales. If not, the applying section will be changed.