Assuming the shot travels at a constant speed... the target is travelling at 10mph which is 52800ft/hr, or 14.67ft/second.
For the shot to go 90ft at 1200ft/s takes .075s.
In that .075 seconds, the target, travelling at 14.67ft/second travels 1.1ft.
That would put the target in the center of the shot pattern if you lead by 1.1ft.
For 20 yards, the shot travel time is reduced by 33.3%, so the lead is reduced by 33.3% which makes it .73ft.
But the shot doesn't travel at 1200fps, that's the max speed. It takes some time to reach that speed and then it starts slowing down. Also the pattern doesn't all reach the target at the same time.