Oil level isn't an exact science. When the car says "Oil low, +1 quart", it is fine to add a quart.
When doing an oil change, I put the dirty oil back into the containers as I use them. This way I know I put in the same amount that came out. Normally this leaves me a little lower than "full", so I keep an extra quart in the trunk and add it when I get the +1 quart message.