Oh, well, those of us who deal with multilayer boards regularly use
stripline: lines sandwiched between ground planes on inner layers. But
from the sounds of it, Wim almost certainly is using microstrip.
However, if you look more carefully at it, you'll discover that your 75
ohm line is NOT 75 ohms "from DC to daylight" -- things fall apart at
low frequencies (significantly so below 1MHz or so), and where the
dimensions of the line are comparable to a wavelength (well below
"daylight"). But again, Wim should be fine using the dimensions you
originally gave him for what he wants to do.

Cheers,
Tom