What percentage should a cancellation fee be?

There is no regulated number here, it is a business decision. A lot of pros charge 50 percent of the service price for a late cancellation and 100 percent for a true no-show, on the idea that a slot canceled with some notice can sometimes still be filled.

If you already collect a deposit at booking, the simplest version of a cancellation fee is just keeping that deposit instead of refunding it. That avoids a second awkward charge after the fact.

Whatever percentage you pick, put it in writing where the client sees it before they book, not buried in a confirmation email they never open.

Turbo Link lets you set a deposit per service and choose whether it is refundable, so the cancellation fee is really just the policy you already collected on, rather than something you have to chase down later.