They probably don't put the weight on there because it's irrelevant. The pellets weigh what they weigh, and your choice is how many vs. how much. It might also be a safety issue as well. Pellets burn different than loose powder, and what might be a safe load with pellets might be an overcharge with loose powder if the same volume were used.

My suggestion would be to stop using pellets and use loose powder. Then you get to control the charge and make it suit your rifle and bullet. However, if you are really set on using pellets, then I'd recommend you not sweat how much they weigh, or their volumetric equivalent to loose, and just drop two down the bore, load the bullet, and pray the rifle likes the load.

Mike