Chalk string would work, but just like I used to ask my students who layed out on metal with a sharpie point vs a scratch awl- which side of the sharpie is the right dimension? Sharpie, as the chalk line, adds thickness to the line.
If I did it that way I.D rather use something like 4 lb fishing line instead
Well, if half the thickness of a string is significant, then yes, but that's a pretty anal approach (not a criticism). Heck, I've installed more studs and pic rails from simply eyeballing it, and I have rarely been off enough to notice. And, with the type of bipods commonly used today, like my Acu-tac, it frankly doesn't make a practical difference, as the rifle can be rotated slightly if necessary to square it. And for field use, the average terrain is so far off plum, it matters not at all. I'm not recommending that folks abandon a more precise method, as my eyeballs have gotten pretty good at this over 50 years of shade tree gunsmithing. And, I admit that merely the knowledge of a job well done is worth something.
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