Tempo to Transition Ratio
Tour pros maintain a 3:1 ratio (backswing to downswing time) regardless of absolute swing speed. Calculate your frames.
Your Target Downswing
frames
To achieve the Tour average 3:1 ratio, your downswing from transition to impact should take exactly this many frames.
The 3:1 Rule
Without the calculator, simply count the frames in your backswing video and divide by 3 to find your ideal downswing length.