patch panel question

[srh3trinity]

I am going to try my hand at making a patch panel and I want some advice. I bought some 16 gauge metal. The rust is on the front fender behind the wheel well down near the rocker where a piece of trim was. How far up should I come to splice the part in? Try to find a flat part of the metal and butt weld it in or do I take it all the way up to a body line? Thanks in advance.