Allowance for framing out of square canvas?

[Traeger-Cowling]

Hi Friends,
A question that comes up occasionally:
Does any one have a formula or rule for how much extra tolerance you need to allow for framing an object/painting that is out of square and can't be squared up?
In this exact case its a stretched canvas 1880 x 1880. Its about 5mm out of square. the left side bows to a small degree, otherwise the sides are straight-ish.
Its an old work on its original strainer it can't be tapped out or restretched. Its going in a bespoke aluminium box frame that will cover approximately 2cm of the edges of the canvas.
Usually I'd add a 2mm tolerance and it would be fine for a work that's in square. In this case given that its 5mm out of square how much bigger would you make the frame? I need to keep it as close as possible to the edges. Any sure fire way to calculate this?
This comes up occasionally and every time its a laborious calculation with a bit of wing and prayer..
In this case I must get it right the first time, It's got a short time frame and it's a costly frame!
Surely one of you boffins have figured this one out?
Many thanks.

[Not your average framer]

You may need to add some extra clearance inside the frame and add a flat slip to hide the fact that the canvas is not square. It's quick, easy and simple, problem solved.

[Traeger-Cowling]

You may need to add some extra clearance inside the frame and add a flat slip to hide the fact that the canvas is not square. It's quick, easy and simple, problem solved.

Thanks very much for your response. Yes indeed, its all about the extra clearance!
I agree, I was just hoping someone had a handy formula, i.e. if its 5mm out of square you need to add___mm to the frame dim
I'll do the work, laty of me to try and get you to do it

[Justintime]

The allowance of 3-6mm around the canvas that I have read, is allowance for future stretching, which doesn't apply here.
As Mark has said, I've used slips which when completed look part of the frame.
I've been dealing with the same this week, such a time drain...

[Traeger-Cowling]

The allowance of 3-6mm around the canvas that I have read, is allowance for future stretching, which doesn't apply here.
As Mark has said, I've used slips which when completed look part of the frame.
I've been dealing with the same this week, such a time drain...

Thanks very much. Its one of these simple difficult things
I think we've figured out how to proceed.
But it looks like now the client may allow more time and a different spec is possible

[prospero]

It's most guessology. A metal frame is a bit of complication as you can't 'nibble' it.

I would work out the 'sight size' using strips of whatever-you-have-lying-about laid on the face of the painting
and jiggled about until you have the max image area but square. Measure and use that measurement to work from.
Don't worry about it being too loose as you can wedge little foam core spacers in to make it sit right in the frame.

[Steve N]

I normally add a slip to the frame if the canvas is out of square, just lay the canvas on a cutting mat with a grid, line up the base of the canvas to one line and corner of the grid, yo can then see how much out of square it is and get a accurate measurement for the frame and slip
As for a for a formula, basic trigonometry, but you need to know the angle, but you can work backwards if you know the lengths and how much it's out, but as I don't do any work for NASA, I'm not into 'Rocket Science'