The Chemandy calculator should be OK but you might like to try this free 2.5D Field Solver for comparison: [http://mdtlc.sourceforge.net/](http://mdtlc.sourceforge.net/) There are several other free solvers available. Have a rummage through: [https://cecas.clemson.edu/cvel/modeling/EMAG/free-codes.html](https://cecas.clemson.edu/cvel/modeling/EMAG/free-codes.html) A lot depends on how accurate and repeatable you need the impedances to be.  Coplanar waveguide is arguably more affected by track and gap dimensional tolerances and less so by substrate and copper surface roughness parameters than microstripand stripline so you may need to ask some more detailed questions of JLCPCB about their line width, gap as well as prepreg thickness and dielectric constant tolerances.