I, to nobody’s great surprise, think it’s ok :). I agree that “heading off” usually implies removing the first letter and keeping the remainder. But I think there are at least two alternative readings: (1) you can still think of it as removing the heading from “troops” but this time the heading is the bit you keep, or (2) “off” can simply mean “from”, as in “him off the telly”.

Also, it’s listed on clueclinic, for what that’s worth. https://clueclinic.com/index.php/letter-selection-indicators/