Thanks to the genius of Mrs HF, my HR queries are solved and I now have three numbers to guide my training from as soon as I can start running again.
Here's the method she used:
Assume HRmax = 180 and HRrest = 48.
180 - 48 = 132. This is the heart rate range. To work out the training zones you take the required percentage of the range and then add back in the HRrest. So...
Long slow steady training at 65% = (65% of 132) + 48 = 134
Cardio/aerobic training at 75% = (75% of 132) + 48 = 147
Interval/anaerobic at 85% or above = (85% of 132) + 48 = 160
Now all I've got to do is to get healed enough to actually run. I've had a bit of a set back while turning over inadvisedly in bed, so it might be a week or so yet.