Fitting Window Sill - Wall not Parallel

[Gizzydash]

Hi All,
Probably a simple question but driving me nuts. Fitting an interior sill, the gap at either side of the sill (wall back to window) is different by about 20mm.
If I fit the sill butted to window, cut at same depth right across, I end up with the overhang and sill horns different at each end. This is noticeable against radiator underneath - looks like something is running off.
What's the solution here, do I ensure overhang at each end is the same and therefore scribe back of the sill to the window or do you just do it parallel to window regardless and end up with different overhangs at each end?

Thanks for any advice!

 

[jacko555]

Scribe so it looked even

 

[Gizzydash]

Thought that was probably best course of action but wasn't fancying a tricky scribe with all the awkward angles to account for, but thanks for advice.

 

[Deleted member 174758]

You always work to the wall line of the opening, i.e. the front (inside) of the window board runs parallel to the radiator so that the "ears" are the same size. If the window isn't parallel to the wall, scribe the back edge of the window board to it