let's think sideways for a second.
You probably have 3 different plot paper sizes and perhaps 3 border template sizes and several potential scales ( but only one scale per drawing)
I'd make several multirow scale bar blocks ... one row for each likely paper ; each row notated something like 1:10 @ A3 , 1:10 @ A1 etc
Just set it up correctly and there will be a scale bar that measures correctly irrespective of the paper size
... without having to modify the drawing.
.... and it will work correctly for PDF plots as a bonus.