A good paper which will give you an insight into the solution is "Study on equivalent circuit of coaxial line to post-wall waveguide transition in 60-GHz band": Microwave and Optical technology letters from Volume 52, Issue 3, pages 623–626, March 2010

DOI: 10.1002/mop.25020

Thank you Mr. John Sharp

R. E. Collins, Foundations for microwave engineering, 1st Ed. Page 164.

Another possibility is to measure the transition. It can be done without only three shorts.

With three shorts

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