Best thing I ever bought tool wise is a digital vernier gauge, measure it accurately then find the seat post to match, from past experience I found a shim just allowed it to move, nothing more annoying than a seat post than turns while riding, if you have a 22.5 tube then find a 22.4 post.
you can get in old school,
22.2 all makes
22.4 kuwahara cro-moly
22.5 haro cro- moly fluted
22.6 hsin lung cro-moly