: i've had the same experience with a large strongback that i was sure was
: strong and rigid. every time i lined something up, something else was out
: of whack - and this did not make any sense - one should be able to get it
: better and better.

: so i pulled everything apart and tried flexing the strongback and noticed it
: really bent at one location. so i took it all apart and there was an
: almost total failure across one of the strongback pces. (this was a
: 6"x3" x 5/8 plywood internal s/b). remade the s/b, and things
: started working from there.

: if you have a good rigid base (s/b) to work from, things will work out.
: be patient and find out what the prob is first.

: everything starts with a good foundation.
I had a similar problem setting up the forms for my Outer Island. I used strings and even a laser and when I was sure it was aligned i started stripping. After about four strips I noticed that the strips around the #9 and #10 form were for some reason bowing out, so...I loosened those two forms and letthe force of the strips move them and everything looked fair from then on. Not sure where I screwed up but I wasn't about to peel all that expensive wood off and start again.
I never spent as much time aligning the strongback on the Guillimot I built previously and it seems to handle just fine. I think if you are within 1/16th or even 1/8th inch you;ll do fine. Remember, you will fair the boat before glassing and how much more of a change in the hull will you introduce then? Skin on frame boats flex with every wave and they perform great or as they say in my other hobby - Relax don't worry, have a homebrew!