Try 3.25 to 3.5 khz (or 3250 - 3500 hz). You can always narrow it down, but in that range you can still understand the speech being inverted, just narrow the frequency to that exact invert point.
And yes SF, perth, and gan all use straight inversion.
Try this
Smithsfalls 142.305 invert freq 3.3 khz +/- 200 hz
Perth 142.125 invert freq 3.3 khz +/- 200 hz
Gan 142.695 invert freq 3.3 khz +/- 200 hz