"When you're extremely light, ie. positioning an empty aircraft, what's your preferred landing flap? And why? (Assuming runway length, weather, wind etc. isn't a factor)"
I've tried F30 & F40 in these conditions. My initial thoughts were F40 because at light weights the N1% is about the same as F30 i.e. mid 50's. This would give better engine response for speed control. It is definitely better if there is a tailwind otherwise the N1% would be well below 50%.
At F30 I expected N1% to be too low and not give such easy speed control, but in fact it was not too bad.
Suck it and see.