I have recently been using the scale located on the right hand side of the plotting chart (in GWX 3.0). I came across its purpose as i was flipping through the GWX user guide manual located in the documentation folder of the SH III directory.
I had never bothered with it, relying on the 3.15 mins. system instead but, after trying it out I actually prefer it today! Sometimes I miss the time intervals between readings because i'm too busy howling at the moon so, this way I take measurements every 5 mins. or so and, i can still get better target speeds without the stress of constantly checking the stopwatch.
regards,
woolfman