I think it's more of a guideline to make people reduce the amount of pellets they feed, rather than a fixed recommendation. So an eggcup is much better than a bowlful, for instance. Hay / grass are far more important as the main component of a rabbit's diet and pellets should be fed sparingly. Some people don't feed pellets at all. I use them as bribes to get them back into their hutches, and as a useful indicator that all is well, so uneaten pellets here are a sign that something is wrong, as they would normally be eaten within a few minutes.
I feed a small amount of pellets once a day using a small jug as a fixed measure for all the rabbits. I would suggest finding a scoop of some kind and using that for consistency. eg the lids of some roll-on deodorants are about egg cup size.
Rabbits vary in size - so a 1kg rabbit should be getting a much smaller volume of pellets than a 3kg rabbit. Some pellets are smaller and more dense than others, so even a fixed volume will give different weights of pellets for different brands. Look for something that suits you, and provide plenty of hay.