Regarding your string from fork tips to DOs, how do you know the steering is centered exactly? As you swing the steering back and forth the fork tips move laterally, so if the steering isn't exactly centered the fork tips won't be centered on the frame. The effect will be bigger for the strings to the DOs than for the strings to the seatstays. This is because at the point where you would measure them (i.e. the distance to the ST) the non-moving attachment point to the seatstays is so close. That is the reason for jiangshi's comment above - leave the steering out of it. Just worry about the main diamond vs. the rear triangles. The fork will still be just as straight (or not) as it always was.