scribing to a wall?

[ColJack]

I have to rip down a piece of plinth which I am using to box in the end of a stud wall, but the wall it's going against isn't flat.. it bows a bit in the middle.
how do I scribe the line to make it fit the wall while still keeping the front edge level with the stud wall..

 

[joinerjohn]

Pin the plinth lightly so you have an equal overlap at the front edge when the back edge is just touching the wall where the bulge is. Measure the overlap and cut a piece of timber the same width as the overlap, but only about an inch long. Hold this against the wall and mark with a pencil whilst sliding this piece down the wall. Cut to the waste side of the pencil mark then fix plinth to wall/ boxing in. Should now fit the wall perfectly.

 

[ColJack]

cheers, I knew there'd be a trick to it..
I was thinking something along the lines of a pair of compases set to the right distance but wondered about keeping them square..