Tom
Just another thought for you; as I have already mentioned, for the Hawker, the manufacturer's recommended flap retract speed is VFTO, not V2 plus 10, for a normal take-off. One very good reason for this could be that V2 plus 10 is usually reached before the gear has started to retract. By the time the gear is fully stowed, VFTO (approx V2plus 30) has been achieved and the aircraft can be cleaned up.
The question now is, what speed would you pitch for if an engine were to fail clean below 1500'? The charts give profiles for a climb to 1500' at V2, however it is unlikely that you have calculated V2 flap less if you have just taken off flapped. Of course, you could approximate or continue at VFTO, however neither method would give you a provable second segment gradient.
Hence, it is far easier for operators to mandate a height of 1500' for flap retraction to allow for an engine failure at any stage prior to that point. All that would then be required would be a pitch to your calculated V2 to maintain a gradient guaranteed to keep you safe to 1500'. Or you could just take off flap less if your type allows it.
AP