SPICEing The Inductance of a Trace Over a Ground Plane?

I see it over and over again...use a ground plane...use a ground
plane...nag nag nag...
I've yet to see a number of how far away that ground plane really is..

I was playing with this:
Wide Trace over a ground plane calculator on
http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=util_inductance_trace_g

I put in:
w = 20mil trace width = 0.000508meters
t= 1oz copper = 0.0014in = 0.0000356 meters
h = 0.8mm = 0.0008 meters
ur = 1

I get an error! Fk...??
Perhaps the ground plane is probably too far away and the trace acts
more like a straight wire inductor and less like a transmission line?
That or bad coding..

The calculator does work for 0.5mm ground plane to trace spacing.

How are designers selecting how far away the ground plane should be?
Anybody have some frequency and distance examples?

I'm trying to spice trace inductance over a ground plane.
D from BC
.

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