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.
I currently weigh between 195-198 lbs @ just under 6'1". I haven't back squatted in a long time, but I have ATG front squatted 285 x 4 recently.
And I agree. I set my SL squats up so that I would be able to use a higher load, but still keep it light enough so as to keep my spinal loading minimal. I've only done them twice, but balance really doesn't seem to be an issue. I like them much more than any form of barbell squat. They hit the glutes and adductors much harder.