Simple:
1 - Clean out the seat post. Do it right. Take out the bottom bracket and clean and even sand it out.
2 - Check its round. Very important, at the midway point and its top. The top of the seat post is likely messed up.
3 - Take a pipe and wrap tape tape around it till it fits snugly in the seat post mid way to the top.
4 - Measure the diameter of the tape and thats what ya need.
Don't be surprised if the diameter does not meet the expected specifications. That happens allot. Also note that this is not, The Way but just, A Way of doing it. I always shoot for a seat post diameter well below the top of the seat post.
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No matter where you're at... There you are... Δf:=f(1/2)-f(-1/2)