What's your back squat max and bw?
Also, the discrepancy may be in the methods…pistol squats are quite a bit harder because the extended leg effectively makes for a heavier leg (due to increased moment arm). This could throw off my calculation. If the leg were just dropped down as in your video then perhaps the equation might be closer for my athletes. Also, having the external loading both in front and behind (as in your video) would make the exercise slightly easier because the weights would counterbalance slightly.
ELITETRACK Founder