I know that there are several members who have experience in IR thermal imaging and I'd like to ask what working distance they use in the calculations?

Depends if you are talking about a facility or a IR service company. For an installation that has done an arc flash study you either use the assumed distances form the study (Usually 18") and wear the right PPE for that or have a special label done for IR scanning, I have seen this at 36".

The 2009 NFPA 70E has added a special task for IR scanning that allows the person doing the IR to be in reduced PPE, the person pulling the covers would follow the "removing covers" task HRC's.

Better yet, install IR windows or a continous IR monitoring system, I know the IR monitoring is big over on your side of the pond.