It's important to calculate based on the finished volume you are bottling so if you brew a 5 gallon batch but only bottle 4.5 gallons then that's what you prime for.
.75-1oz priming sugar per gallon is a good reference to start with.
I hate to go straight to the C-word but never rule out the possibility of contamination. Back when I was bottling I kept getting fizzy bottles, but I was sure I had used the right amount of priming sugar. In retrospect I realize that my sanitation wasn't perfect, and I was blind to the notion that MY (perfect) beer could EVER get an infection. Wrong!