Modified cossack squat idea

[Vilgefortz]

I'm trying to make some pancake progress, im 110-120 degrees at the hip and terrible side splits flexibility atm and straddle good mornings work very slow for me so i tried cossacks. They worked great in the past, but after this long break, the cossack squat gives me knee pain and I couldn't perform full ROM due to lack of flexibility on the stretched leg anyways. So i though why not elevate the squatting leg on a chair. I can go full ROM and keep a 90 degree angle between the direction of the bent leg toe and stretched toe as Mr Kit instructed in a youtube video. Any reason this is a bad idea? 

[Kit_L]

No, it's a good idea. Anything you can do to achieve the movement pattern is fine.

[Vilgefortz]

Of course you thought about it a decade before  Thanks! I'm using a chair twice the height since even that causes pain.