Backbow on my neck.

[PeMe]

Im building the 12-string tele, so hopefully the tension of the strings will fix some of the backbow. but the backbow is so big, when i put my straightedge (ive notched one side of a 50cm piece of machincrafted metal) on the neck and put it flush at the nut there is a 3mm gap at "the end of the neck" (after the 21th fret) and a 1 mm gap between the 12th and 13th fret.
You guys think that the tension will straighten all of that?

[Dikkybee007]

I normally adjust the truss rod so when I use my notched straight edge the fingerboard is dead flat, or as close as I can get it, so I can level the frets. I have had one neck that I could not get straight so I removed the frets and then sanded the fingerboard level and replaced the frets. Once I level the frets and put the strings on it normally bends the neck to give the correct relief. I know you would get more tension if it is a 12 string but I don't think you would remove that amount of bend but I could be wrong. I normally set mine so when I put a capo between the nut and first fret and press the string down on the last fret that there is about 5 to 10 thou clearance but if you cant get it flat to begin with I don't know how you are going to get the frets flat. If you cant get the frets flat then you have to have a higher action so you don't get buzz. My personal preference is to have the neck as straight as possible so I can set my strings as low as possible but that is my preference as I like as low as possible action on all my guitars.