Unless the PDA is coupled with a reliable pressure altitude instrument upstream (such as a certified, calibrated logger), the PDAs own pressure altitude is going to be pretty poor in terms of accuracy. PDA or mobile device altimeters, especially and old dell streak, are about as reliable as a cheap altimeter watch. In that case (I also had a dell streak and XCSoar for awhile), I do use GPS as a reference and relied on GPS more than pressure, often cross referencing with my steam gauge altimeter to see how far it's off.. In other words, I flew the "worst" of the two in terms of final glide. If one altitude says I have final glide by 300 feet and the other says 200 below, I assumed the worst. I also turn up the fudge factor (add safety buffer). Also, the best final glide profile is generally to build up the margin early in the final glide and then begin diving it off at 5-10 miles depending on the glider performance and conditions. So when using a low accuracy PDA pressure altitude I would hold off longer to burn off any excess altitude and be weary of the potential benefit of risking a low finish. In other words I often finished fairly high.

Obviously, in US and FAI contests, the scoring software adjusts pressure changes and compensates. So the altimeter setting you launch with is very accurate at the finish assuming you have a fairly reliable, calibrated pressure altitude. A PDA without reliable pressure altitude is going to be fairly poor, to almost useless.

Even with a quality pressure altitude instrument, altitude considerations and learning to trust your final glide accuracy, and to understand and trust exactly what your logger is logging as you get close to min altitude, takes some practice. John Godfrey is excellent at explaining this as is John Good.

Post my XCSoar experiences, I have had an SN10/Oudie and now a ClearNAV2 and LXNAV S10. These instruments are all rock solid in terms of pressure altitude and I have quickly grown to trust them completely. They are both very accurate and reliable in terms of contest final glides and accuracy. I can usually take myself down to within 15 feet of the minimum finish altitude in total confidence. Also, the pressure altitude displayed on the screen is (generally) almost exactly what the logger is showing at that moment. So, if you see 1003 and the min is 1000, your OK, generally. Yes, I know the CN2 is expensive but the SN10 is now pretty cheap and very easy to install. And the SN10 NMEAs nicely to the PDA of your choice if you want a moving map to "supplement" the core SN10 data.

Not sure that really helps, but this is a quick summary of my experience.

S-