tumor marker reading

Can anyone tell me what the reading is on a tumor marker? I mean, what's the lowest reading and the highest? I asked my onc last week and she said it real a "12" which she said was good. She was in a bad mood and I was too preoccupied with it to ask what's the best reading and the worst so I something to measure it by. Now it's bugging me and I won't see her for another few months. Thanks, Linda