Interesting post.. for all my inquisitive nature I'd never considered the derivation of the constant. I've unquestioningly used 450240 ever since I first read it in Airgunner (

) at the age of about 15

Rob; as you point out this issue arises from varying available values for gravitational acceleration; would it not be easiest to work through the SI formula converting to imperial, using the

standard gravitational acceleration constant of 9.80665m/s^2..?
As you rightly point out the potential for inaccuracy from such variations is pretty small - at this point I'd start worrying more about the accuracy of my measuring equipment, rounding errors etc