Thanks Andreas,
I gave your suggestion a try. Unfortunately final cut doesn't recognize the key as being held down - just as a tap.
I did use your method to set it to switch back to the default tool on mouse up, which is working about 50% of the time.
Here's my file.. not sure if I can improve it.
fcp range tool.bttpreset (7.3 KB)
For context, in final cut you hold down the R button + left click and drag to select a range of a clip. Trying to move this all to one button on my mouse.