Yes, the line spacing can be specified in %. The line spacing is saved in uedit32.ini with 2 settings at [Settings]Spacing Multiplier=
The line spacing factor used is calculated by Spacing Multiplier / Spacing Divisor
Single Spacing is: Spacing Multiplier=1 and Spacing Divisor=1 - result is 1.00.
1 1/2 Spacing is: Spacing Multiplier=3 and Spacing Divisor=2 - result is 1.50.
Double Spacing is: Spacing Multiplier=2 and Spacing Divisor=1 - result is 2.00.
1 1/3 Spacing is: Spacing Multiplier=4 and Spacing Divisor=3 - result is 1.33.
1 1/4 Spacing is: Spacing Multiplier=5 and Spacing Divisor=4 - result is 1.25.
You can only enter integer values for the 2 settings. And you cannot edit the 2 settings in uedit32.ini with UltraEdit itself when it uses this uedit32.ini because it will reset the 2 values when closing UltraEdit. You either create a copy of uedit32.ini and modify the 2 settings in the copy, close UE and replace the original uedit32.ini with the modified copy or you use Notepad to edit uedit32.ini while UltraEdit is not running.
Where is uedit32.ini? Since UE v12.20 you can see it in the configuration dialog at Application Layout - Advanced - INI File Location
or use the infos posted at UltraEdit INI file location
to find uedit32.ini.