Ok, Petr, thank you for the detailed feedback.
On my HP HDX18
running VISTA X64 and using the Sky background in full HD
mode (1920 x 1080),
it ranges between 18-24, whatever the stretch mode being turned on or off.
This means, better to use the full screen mode with a horse race powered computer
As a matter of comparison, on the same computer, MovieBox, playing a full HD MKV movie, ranges between 27-36.