I downshift at about 2500-3000 RPM, I'd assume it would be the same in the 7AT in manual mode? It was on the F430...
Just practice, as others have said before, you'll get the hang of it and learn when to downshift and upshift and for which situations. Plus, if you ever get tired of it, you can switch back to auto mode. Me, I have to sit and shift through traffic... 1st - 2nd - N/stop - 1st -2nd - N/stop... god damn traffic!