I don't think I'd trust the way you are measuring.

I think a better way to measure if there is a significant bow would be to make four centerline marks on the fuselage. One line at the firewall, one line in front of the wind saddle, one half way between the wing saddle and tail and one line near the tail. Stretch a piece of string between the front and rear marks and see how far off the string is on the two in the middle.