What you really need to start is the dielectric thickness and dielectric
constant for the material you want to make the board with.  As some
mentioned, FR4 works well at 2.5, if you don't have long lines, or can
live with the extra loss.  FR4 is also much cheaper than the higher
frequency materails.1/2 oz copper helps maintain closer line tolerance
during etching, and current handling usually isn't an issue sense you'll
be plating up for all the ground vias.  Don't forget, the solder mask
changes the dielectric constant, so your calculations will be more
accurate if you can remove the solder mask above the 50 ohm lines.

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