39 will lead to the answers they have supplied. Anything other than that may actually leave you with an answer that's between two of their options.
Also, be careful not to fall into the trap of attempting to be unnecessarily precise with your calculations. The rounding and interpolation principles are available on the CASA website and you need to stick to these. Sometimes, people try to leave all the rounding until the end. It is more precise, but it is not the way the CASA answers were produced.