Built the strongback to the full 20 feet on a concrete floor. Then put it on horses about 30 minutes later to get ready for forms. Then I cut it to length. Put it on horses and It is not straight. It has a curve at the one end and is off by about 1/2 inch. I am a perfectionist so now I am trying to reshape with weights and clamps.
Am I being a perfectionist and am I okay if it is slightly off.
If off now won't the strips exaggerate the curve in the strongback

I have given this some thought. I have an internal 2" x 4" strongback made of 1/2" plywood. in the vertical dimension it is dead straight (within 1/32" over the length) so I do not have to worry about adding or removing rocker to the hull. However it has an 1/8 bow in in the lateral dimension effectively putting a curve in the keel. I have dealt with this by cutting the slots in the forms about 3/16" wider and using wedges and business cards to shim up the center-line of the forms on the strongback.

I am thinking of removing the lateral curve by routing a 1/8"x 1 1/4" dado in the side, then laying it down on my external strongback such that it is dead straight and epoxying in carbon fiber tow on each side. I decided against using an aluminum extrusion, because I can't modify it, screw into it or staple into it which I often do.

IMHO 1/2" sounds like an AWFUL lot to me. If you start over now, while your investment in time and material is minimal, you will be better off in the long run. As Marc has said: get a laser level. They can be had for a few bucks and I guarantee it will be dead-straight.