The in camera meter (measures light reflecting off of the object) measures for 18% gray, when you take a picture of something white and the meter reads in the middle the white object will be 18% gray, if you expose for 1 stop higher it will be white with detail, if you expose 2 stops higher it will be white with no detail (blown out) -good for white seamless backgrounds, that is what the zone system explains
I just took a lighting class in school and practiced/learned ratios and the zone system, I also use a light meter to get correct exposure too by actually measuring the light falling into the subject (incident), I have been doing product shots lately
Also it would probably helpful if you look at your histogram as your shooting, then you could probably get a better idea of your quality/amount of light that is reaching the camera sensor