Yes, I know: you have cork under your ruler.....
BUT.....Cutting against a metal straight edge will work well, until you cut a big piece for an expensive project.
And then the straight edge will slip and you will cut into your project leather...... [Learning from a friend...]
So I suggest this:
Use a ruler / straight edge and a scratch awl to mark the line where you want to cut. Then make the cut freehand!
Because during the marking phase you can focus just on keeping the ruler straight. Then when you do the cut, you can just focus on that and do not have to worry to also keep the ruler stable.
Here it is in action: