I think around 90%....
I think around 90% of my clients desktop computers for the staff could be happily shifted across to ARM now. The PI is an excellent example, it'll run day to day stuff, cheaply both in investment and ongoing running costs. I'm actually doing the maths at the moment on if we replace a 25 seat environment, are we saving more money buying PoE Hats for them rather than powering individually, or each powered off a monitor's USB.
Any heavy lifting tasks my clients do on bespoke can be re-coded into web apps running on a local server now. Most of the legacy stuff is going to require a rewrite anyway, so might as well just bite the bullet and make them local intranet apps.
I see little need now for x86 outside specialist applications or the server room... Certainly no need for Windows.