I'm sure Reg's programs will give you a very good estimate. For a
quick "rule of thumb" estimate, assuming ~77ohm air-insulated coax with
an inside diameter of the outer conductor of D inches, at frequency f
MHz, Qu will be about 100*D*sqrt(f). So a 1" line at 100MHz will give
you Qu about 1000. This quick estimate may be all you need, and even
quicker than loading a program.

I trust you are aware that the Qu won't tell you how sharp a filter
will result; for that, you need to know the configuration and the
loaded Q--or better, where the poles and zeros are. There are lots of
ways to do that; RFSim99 might be a useful (and free) tool for you. By
appropriately tapping the thru-line down on an appropriate stub, you
can arrange to pass one specific frequency with very little disturbance
while blocking one other specific frequency of your choice (and another
you don't have so much control over, and of course others at multiples
of the lowest freqs). Or you can combine a stub that blocks a
particular frequency with a reactance (capacitor or inductor) that
tunes out the reactance of the stub at the frequency you specifically
want to pass.

For example, put a 77-ohm shorted stub that's a half-wave long at
125MHz across your line, and you notch 125MHz. Put an 18.8pF
capacitance across that, and you will pass 150MHz with little effect,
because the 18.8pF resonates the inductive reactance that the stub
represents at 150MHz. But it will also pass 46.5MHz with little
effect. Put about 27nH across the same stub and you'll notch 125 while
nicely passing 115MHz. Include line and inductor loss in the model to
see the depth of the notch and the attenuation at the "pass" frequency.

Cheers,
Tom