Most of my thinning is with percussion, & overshot flakes can be a good thing. Even if they are an accident.
I used to like them, but have started losing a lot of width to them that I cant afford to lose this early in my knapping "career" haha.
If I could control them, that'd be different. The latest one that got me that upset me the most was out of a piece of Cowboy G-town. Was thinning it down and took a deep bite out of the middle with mass left on both ends. I thought I was going to snap it for sure. Especially when I came in from the other side to smooth out where that deep flake rolled out. Got it thinned and back under control and was just trying to use a small hammer stone (what i did most of the work with) and was going for some mass right at the end of the biface on a near vertical platform. With that hammerstone, I generally swipe down (probably the wrong thing to do) across the platform, more or less brushing it to rip the flake off rather than "hitting" it per say. When I did the flake detached though the middle (thickness wise) and then rolled out the top about half way down the biface. I'm not sure if that makes sense to you or not. I went back and watched Jim Winns Hammerstone tips series and looks as if he goes for a high hit on a steep platform. Not sure how it works for him, but it leaves deep bulbs of percussion for me.
Anyway I wont discuss any more of my problems in your threads, Ive already said too much to take away from the beauty of this point.