I was once burned by an 8 KHz clock which had 1 ns risetimes. The culprit
wasn't a 40 kHz harmonic, but the energy up around 350 MHz.
I use the rule of thumb arrived at by driving a step input to a simple first
order R-C low pass filter whose bandwidth is:
B = 1/(2*Pi*R*C)
When the filter's output signal 10% -> 90% risetime (tr) is multiplied by the
tr*B ~= 0.35