How is departure tax calculated on private company shares that don't have an easy market value?

Private company shares get caught by the same deemed disposition rule as any other capital property, but they're harder to deal with in practice because there's no stock exchange price to point to. You still need a fair market value for the shares as of your departure date, and arriving at that value generally means a proper business valuation, looking at the company's assets, earnings, industry, and comparable transactions, rather than a rough guess based on what the company might be worth someday.

Because the valuation is the whole basis for the resulting capital gain, or loss, and because private company values are inherently more debatable than a public share price, this is an area where CRA is more likely to take a closer look if it's ever reviewed. A defensible, professionally prepared valuation, done around the time of departure rather than reconstructed later, is your best protection if the numbers are ever questioned.

If the shares are illiquid and you don't want to fund the tax bill from your own pocket before you've actually sold anything, the deferral election covered elsewhere becomes especially relevant here, since it lets you push payment out until there's an actual sale generating cash.