FR-4 has Er around 4.6, and 1 oz copper is 1.4 mils, so I use those numbers and impedances seem to come out close. It seems to me that the cheapie multilayer houses (in the back of EE Times and such) often prefer 12 mil dielectrics on the outer layers, about 30 for the inner, which is sort of silly but will give you 75 ohms microstrip with an 8 mil trace. Could get lossy with such skinny traces.
Probably calling out 20+20+20 with a few mils tolerance on the dielectrics is safe for multisourcing. Lots of board houses will tweak your art to a target impedance, but I don't like board houses messing with my dimensions.
I'm just finishing up a board right now. 2 layers, 0.062 FR-4, 0.72 square inches total. It's a tiny 50 MHz oscillator. The reference designators are 50 mils high, which means most nobody can screen them legibly, so I guess I'll skip the silkscreen step. Last week we did a board that must be about 0.1 square inch.
John