[dba-Tech] Full-Time v. Contract

Does anyone have any useful tips on how to calculate a salaried position v.
a contract? Obviously a contract should pay more, since there are no
benefits (dental plan, holidays, etc.), but I've been a contractor for ages
and lately have been asked if I'd take a position instead. I can see the
merits to switching, but I can also see downsides. Let's assume for the sake
of simple argument that your contract wage is $100/hr, and that the position
offers $x + dental plan, 3 weeks holidays, the opportunity to take a course
or two a year at company expense, etc., not to mention the illusion of
permanence. Fact is, I don't think that I've ever worked 49 weeks in a row
as a contractor. So the simple calculation of 40 * 50 * 100 misses the mark
by a wide margin.

Any rules of thumb? Any on-line calculators for this sort of thing?

TIA,
Arthur