That's a good point FFF. Next time I can I'll do some experiments to see what amount of difference we're talking about. I could also use the other information gleaned here to elect to climb but only up to a max of say 2800' and then stay level. I presumably wouldn't normally have the presence of mind to be so accurate in the event of a serious failure, but after all this discussion and with the mind so keyed up for those few minutes of the flight I reckon I could manage it. Thanks!