It's closer to 10%, but that just makes having a fluid and easy editing experience more important IMO. Editing tasks are interruptions at that level, and you want to make it as easy and quick as possible to get your mental models re-loaded into short-term memory.
I admit, 5% is a subjective prior / fudge-factor. But, statistical white-lies communicate a promise which extends the invitation: try it, see if you're more productive.