It's hard to give direct explination to you without knowing where in the world you are located. For example in the United States you can weigh everything and allow a small amount of under packed product if you can prove the average of the lot was over though you can sill be in trouble due to severly underpacked items.
That's typically how I've seen weights performed at my last two locations. There was a minimum weight and a maximum weight and a checkweigher kicked anything off that was too low. With a checkweigher you can set the weight of the total product with packaging and it will reject anything that is too low. Meanwhile your operators would tare out the packaging on a scale and then weigh the product and with the packaging tore out of the total weight you will know how much is in your product.
For example. If you are shooting for a 500g (+10g - 7g) product you put the packaging on the scale and hit tare. Now let's suppose the scale says -80g once the packaging is removed. When you put a product sample from the line on it anything that is 493 - 510g would be acceptable.
However dependent on location there may be different laws on how much under you may be allowed if at all.