Hi there,

I'm new to plastering and just about to install some 90mm cove cornice.
I got a 90mm plywood mitrebox and found that the cornice is about 87mm, NOT 90mm, which means it leans back a bit in the box and therefore the cut is not at right angles through the material.

Is this how it is supposed to be or should I pack out the mitrebox with a 3mm strip of timber?

I have not tried any of this yet. Am I missing something?
What do professionals do? Just cut and fill up gaps?

I asked at 2 trade centres but were not aware of this.
Literature I read say to mark a line 90mm down wall as baseline but 3mm is enough to throw joints out (?) Or does 90mm down compensate and everything turns out correct?

Any insight into this would be appreciated.

Thanks


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