I have been audited, but that was after we were incorporated. The business rents part of the house from the humans, who declare the rent as income and deduct hydro, insurance etc for that fraction of the house. The written Canadian rules are very clear that you can do it by square feet, by "one room out of 5 so 20%", or anything else that seems reasonable. We don't depreciate the house as an expense since that would mess with our taxes when we sell it.
From talking to other people who do this it seems like reasonable claims will be accepted and unreasonable ones will be checked with a quick audit. If you write on a laptop, you don't need 95% of a 5000 square foot house with only 5% for non business activity, right? (I do know one person who, on getting a very high valuation for his home property taxes, double what he thought it was worth, sold it to his company who then let him live in it at a drastically reduced rent in exchange for being the "super" and maintaining it. I have a feeling he was eventually audited. Don't do that sort of thing and you'll be fine.)
It doesn't matter that you could take your laptop anywhere or even that you do take your laptop anywhere, or that you can take a phone call in any room. While our audit was underway we left the office, which has desks and office chairs and filing cabinets and whiteboards, to go get certain more confidential files that the auditor wanted, and there was no "aha you do part of your work in the rest of the house as well!" moment because that's not a thing. What is a thing is, do you use this space to work in? And if you do, you do.
As to whether you use the room for anything else, if it is your main place of business it is ok to also use the room for something else. If you have a main place of business elsewhere then this home space must be exclusively for the business.