: I'd go at least 1/4. If you have one of those cheap laser pointers you can
: get a quick check with it also. I left the string line in for the first
: part of construction so I could see if anything had changed.

Try 1/2in or more w/ a bullseye drawn across each hole. That way the forms can bumparound a bit without touching the lines(remove drillhole splinters)
and yet you can see well enough to still align to ½ to ¼ the string thickness. And keep the stringlines there for the whole process, why not? It’ll give you
peace of mind during building or an understanding of what’s and how much wrong if problems. (had a scare awhile back, my forms during building all of a sudden became ¼ in out of alignment! After abt 5 minutes embarrassment and dread, I noticed a pc of wood lying across one of the lines – whew!) take a look at the pic, you can see or check alignment from a distance even. (aside - the bumps are now flat from repeated sanding – there was once this girl . . . )

The big key is you must have a stiff strongback.That'll allow freedom and ignore the floor. But by using string, the strongback straightness becomes completely irrelevant. You don’t have to worry about strongback straightness, waviness, twist, surface variations, screw heads and also not worry about exact form hole rotation or tightness smoothness or evenness or centality or whatever. Because you really want loose forms, to align them, to(oh no!) realign them, to slide them on and remove them, and to loosen them if when stapless. Use tight forms only if you are a very good builder.(but there are lots here – not me)

And rather than spending interminable time aligning and coming back and aligning, consider using face wedge clamping. 2 pushes w/ your thumb and a few side taps and each is aligned. How much less do you want? And go and paddle your beater. Or beat . . .

But really any method if careful will work, and has.

-mick