Notice that Gerhard_dk4xp's board does not have ground on the top layer near the trace. This is microstrip. With ground on either side you have a grounded coplaner waveguide which has more capacitance per unit length than microstrip and therefore lower impedance (Zo=SQRT(L/C)) given the same dimensions (width, height). Solder mask has minimal effect. Conductor thickness has more effect than mask, which is why you need to calculate with the finished thickness (after plating). When doing boards with a thin dielectic on the top layer which makes for a thin 50 ohm line, if the pad for the SMA edge launch pin needs to be wider, you can eliminate ground directly under the pin pad to increase the dielectric thickness. I'll calculate what the pin pad width will be (to get 50 ohms) if the ground is on layer 3, or 4, or 5... I'll chose a ground reference layer for the pin pad that gives me an appropriate width for soldering the SMA pin.