I think historically apple has maintained approximately a 10% market share plus or minus 3 points with a higher share for the US and collegiate market.

ChromeOS is supposedly 6% of the desktop market share according to stat counter that I believe confusingly includes desktop/laptop under the same grouping. I'm guessing that about nobody is running chromeOS on a desktop so a reasonable guess is that their share of the laptop market is could easily be 10ish percent.

This would leave windows with somewhere between 70% and 80%

Aside, I've considered using a ChromiumOS for older desktops as a refurb option a few times, and for the likes of my mother in-law who likes desktop form factor, but needs something more "safe" than a typical desktop.