Help with dog food measurements!!

I have been feeding my dog Hi Life Complete Moist which states she is to have 300g. Her weight is 21kg. I bought a measuring cup from Pets At Home and one cup is 150g, therefore she's to have two cups a day. This seemed a hell of a lot and her stomach was solid all night and was groaning as if she was in pain (she's fine today). Can anyone give me an easier way to work out 300g? I have a very expensive food weighing scale and even that is completely different to the measuring cup. Im not the brightest spark so working it out for me in tbsp would be great

A cup is not a weight measurement, it is a millitre not a milligram. So maybe if you weigh a cup then add the food to it minus the cup weight you will have the weight of the food.
Is it in a tin?? If they have the weight on there that could help possibly.
Also have you changed her food brand recently?? As that could upset her gut also.

Emrad's idea is a good one.
Is the measuring cup specifically for the food you are using?
Remember everthing weighs differently, a cup of lead will weigh heavier than a cup of feathers. Like wise I would imagine different foods weigh differently.
Does that make sense

Thanks for your reply The measuring cup i got is marked for cats and dogs and is for measuring dry foods.

It sounds like you need to rely on the scales rather than the cup, moist dog food normally weighs more than dry food

Moist foods are largely water. I am guessing 300 grams would be slightly over an American 8 oz. cup. The cup would be 16 TBSP.

You could note the weight in a can of food, and use your measuring cut to see how many of your cups in a can.

Of course, the 300 grams is only a starting point. Adjust as needed to keep the ribs hidden, but easy to feel.

I weighed the dog food on my scales, then found a container which would hold the 100 grams, it happened to be one of my kids cups and I use that. You could do the same but with 150g?

Every food varies a little in its weight/volume. If you are feeding a dry food then get the measuring cup that matches your food (the manufacturer will almost certainly provide one if your pet shop doesn't)
If feeding moist, then if it's not a complete pack/day use kitchen scales and weight it into a plastic tub (zero the scales with the tub on first)
If you use the same tub every meal time then draw a line around it in indelible ink at the correct level of food, then you don't have to weigh it every time.