LOW OIL light! Help

When the low oil light comes on, how low is the oil. I know that our cars hold 5.5 quarts. I would guess it is about 2 quarts low but I don't know, help me out here guys.

:SNSign:

Well, its a long story but on my way back from Atco I stopped at a rest stop to get something to drink. When I came out I decided to check my oil. Pull my dipstick and it said it was good. Not full but not to the add mark. Just so you guys know I am running a custom dipstick because I needed one that I could bend to go around the main girdle and not get the dipstick caught up in the rotating assembly.
Well anyways when i pulled the dipstick the oil was fine but the stick looked like it got nicked by the rotating assembly. I said I am not going to take any chances and removed the stick from the handle (remember I said it was custom so the stick uses a set screw to hold it to the handle) and put the handle back it to plug the dipstick hole. Well when I started the motor the oil light then came on. I was freaking but I put in another quart and it the light went out, and no other problems.

So basically I am worried why the light didn't come on before, like when I was driving, it would have been lower at the point anyways right. The next big concern is the dipstick said the level was fine and I have been going by this dipstick since I got the car together. Everytime I change the oil and check it, it reads full. So why did the stick read at the rest stop that is was probably 1/2 quart down but was low enough to put on the light. So I am confused and just trying to figure this out.