Allan, 1.6mm FR4 is what easyEDA/jlcPCB offer. I want to try out their service.
According to that calculator, I can use track width 63mil with clearance of 15mil and still hit 50R. Do you think the calculator is wrong?
I don't know what you mean by reference design, is that something I might find in the data sheet for the rfm95 module?
What's "alumina"? An aluminium substrate?