The correct formula (IIRC) is posted by qwerty 2 above.
The time of passing was as good as the relevant ETA's at each of the reporting points concerned, and was usually within a minute, on the relatively short sector lengths in the airspace I worked.
One of my trainers on Area insisted that as procedural controller, I write the ETP for every potential confliction on the radar controllers' strip before passing it to him/her. I became quick at the calculation. I wouldn't remember how to do it now. I haven't even seen a circular sliderule/nav computer for years.
Use of airspeeds is irrelevant. GS is the only measure that can provide the correct figure, hence the use of a formula based on the actual/estimated time at fixed points.
Using IAS will provide the correct value only in nil wind, and if the two aircraft are at similar levels (obv not the same level).
There weren't many times I had to use it in anger. You'd have to be a bit unlucky and have a DME station or two inop. Using 10DME after passing is usually a bit less restrictive than using 10 min.
Back in the day, the guys working oceanic were very good at it. Had to be.