I suspect this version is what you might need.
Code:
cmd: "File.Open_File" image="BtnBottom/features-list.svg" imagechange="true" stroke=1.1 size=50 typ=menu val="str:BtnBottom/Mickey.nc@Mickey|str:BtnBottom/Butterfly.nc@Butterfly|str:BtnBottom/Arcs.nc@Arcs|str:BtnBottom/Square.nc@Square"
I think that will allow you to do something like put in menu options of 10%, 20% etc and then run a gcode script.
As Andrej mentioned the slider stuff isn't complete yet, so you are probably stuck doing it like this for now.