thats true BOAC..
ok mountainlad...
here is ( if memory serves at this stage ...subject to correction..)...a formula..
DISTANCE DEVIDED BY GROUNDSPEED MULTIPLIED BY 60 = TIME IN MINUTEs...hence giving..ETA. we would calculate that either the hard way by using a whizz wheel flight computer...of a pocket size electronic flight calculator..but for rough calculations ( especially in no wind conditions when the airspeed is the same as the ground speed or using an estimated ground speed)..it works fine.
as far as working out the estimated ground speed is concerned again this is worked out using whizz wheel or electronic calculator, based on forecast wind for the route ( since the wind angle relative to the aircraft will have an effect on its forward speed )...and then checked periodically throughout the trip to see if the estimate is correct or needs revision to amend the ETA accordingly..
hope thats of some help..and may the wind be always at your back...
the dean.