Dazzle them with brilliance, or baffle them with bull ****!
I'm building my back bedroom wall with a 42" removable section... I needed to copy the curve of the roof...
I looked at it and I looked at it and thought "man, you're going to screw this up!"
Then I thought about layers!
I layered 4 narrow strips of foil backed styrofoam (left over from another project) between the opening and blue taped it to roof... Screwed a piece of OSB on the back side, and traced the curve! Now, cut out the OSB and I have my template for my 2x6 header.
It's the same curve, just 4 layers lower... Cut the curve, and slide it up!
(I pushed the foam against roofline as I traced it)