I don't think it's quite as simple as just hitting the sweet spot, but maybe? Here's my reasoning:
I forgot why, but for some reason I needed to make a video clip of the glass breaking. I never really tried to break the glass, but I did know that the glass always broke in the same spot, and always while shooting up. So I started playing and just taking shots up, and to the extend I could I kept aiming at that same location. I swear I hit that sweet spot a bunch of times, but it didn't always break. In fact it took friggin' forever to get the glass breaking animation! As I thought about it, it really is quite rare that I see a glass break in a game. Maybe 1/50...75?
I do like that Simmons says it's 3 pixels wide by 5 pixels tall. It's so specific, and from 1993 when this was fresh in his mind. GREAT FIND, thanks for sharing.