Rather than trying to sight down a thread, find a straight edge or ruler longer than 15 inches or so. Lay it on the outer face of the outer chainring edgewise (not flat because it can flex in that direction) and carry that line back to the cassette. You'll need to compensate for the distance from the centerline between the rings (midpoint of the gap on doubles, or middle ring on triples) to the outer face where your edge is, but the correction should have you end up with the centerline dead center of the cassette.

In any case, spreading the frame shouldn't have been that big an issue if you did it evenly, since the cassette centerline moves out half the distance that you spread.