This is an interesting question.When I used to make the odd landing at an airfield around 8000 amsl even with no wind, there was a very big split between IAS and Groundspeed.At the stipulated "60 knots" if you used IAS as your guide the groundspeed could be much higher.The various (Boeing) FCTMs do not differentiate between IAS and GS so to be conservative I used the higher indication and delayed reverse thrust reduction until the GS indicated 60 ignoring the PMs call of "60 knots".Never had any issues.....