1. A serving of meat is 3-4 oz.
2. How you get the meat sliced matters. You can weigh each slice or you can do a little bit of math. If you ask for 1/3 lb and they give you .34 lb ( the label the put on it has that info or just look at the scale when they are measuring it out) then count the slices, and divide it out. Say you get four, then each slice would be .085 lbs or 1.36 oz. But if you got it thin sliced there would be less beef in each slice, so it might only be an ounce a slice. But a thick slice might be 1.7 oz a slice. So you do need to do the math for what you get.
-google first. ask questions later.