The glass is actually crystal (i.e. leaded), so estimating quantity through weight might be a little misleading when compared to normal glass. I still knap TV glass from time to time. I almost always snap my larger pieces in half. The impact wave travels through the stuff with practically zero resistance. I have better luck using antler for percussion flaking, weighted copper seems to do more damage. Good luck. The stuff is almost too fragile, for me anyway. It will definitely teach you to hit the right spot or suffer the consequences.
Keep your eyes open for old toilets. Some of them knap pretty well, some are way to grainy. Once I found what I suspect was a chunk of old counter top that was like white glass. The stuff was amazing. I have seen anything like it since. Actually some coffee mugs are made of a similar material, but those are a far cry from 3/4" thick chunks of counter.