No, I guess you're right. I measured out a cup and it came to about 250grams. So that's the same weight as yours.
However, keep in mind that I'm not changing out the full recommended volume of gfo per week. The RC calculator for example says for my tank volume I should be using about 2.5 cups and changing it out every 4 weeks. Instead I use a smaller volume and change it out every week or two. So it kind of works out the same (not quite, but close-ish). The equivalent in your case assuming a 75g tank volume would be something like 3 tablespoons per week, or 50 grams, I think. I probably have my math all wrong on this but something like that anyhow..
