Groundspeed (kts)
-----------------
200
is a reasonable rule of thumb for Rate 1. Handy for working out the lead distance for turning 90 deg onto a DME arc ie radius of turn. You could substitute TAS in nil wind.
I've also seen 100 used as the denominator but that might be more applicable at much faster speeds.
You could always use the standard radius of turn formula given on the aviation formulary website to derive reasonable approximations. Start with the maximum & minimum speeds & AoB you wish to consider to derive a radius for each then work out what factor for the TAS used gives the same answer.
Depending on how much difference in speeds you use you'll be able to derive a rule of thumb for a particular speed/AoB range.