Forget my question, I found the answer. It's called the "Standard Closing Angle" and Beagle meantioned it up above.
Basically you work it out on the ground:
60/(NMs/Min) = SCA. (SO 90kts TAS would be 60/1.5 = 40deg).
Then turn the SCA backtwards track and fly on that angle for 1 min for every mile, then turn back on track. I think I'll give it a go!