I just had a customer ask me about the tolerance of our measurements. For those unfamiliar, the tolerance is the amount by which a piece can be wrong, and still be considered acceptable. They were comparing to measurements used in carpentry, so I explained that our measurements have to be far more precise.
When working with wood, a tolerance of 1/8 of an inch (or sometimes even more) is acceptable, because wood is malleable, and can be adjusted on the fly. If you need it a little smaller, you can simply take it back to the saw, or use a planer.
With glass, you don't have that option. In order to cut glass down, we have to cut off a minimum of a little more than half the thickness of the glass. And glass is highly unforgiving. If a pane of glass is even 1/16 of an inch too wide, it simply cannot fit. It may as well be a full inch too wide. Actually, an inch would be better, because that could be cut down.
For these reasons, when cutting glass my acceptable tolerance is 1/32 of an inch. If a piece ends up wrong by more than that, it's going to have to be recut. So, when we provide measurements, you can rest assured your glass is going to be precisely correct.
~Fanon