Things like that are hard to eyeball and get straight. Masking tape or a sharpie (water washable) marker and a cloth measuring tape, string line it, have to lay it out some way to guarentee that it'll be straight.
I know what you mean though, not quite 1/8" off and everyone says "looks straight to me", but it's not, and it'll drive me nuts, because I can see it. The curse of being a steel fabricator.