Scales MUST be calibrated, and checked often. Otherwise they are useless.

 

I use small weight-lifting disc weights put in a plastic grocery sack and hung on my scales. I have 2-1/2lb and 5lb weights, which can allow me to check my scales at 2-1/2, 5, 7-1/2, 10, and 12-1/2lbs.

Yup, this is the answer. Easiest way I have found to make sure scales are reading accurately ( and I have many digital and spring scales available at all time) is to keep a 5lb cannonball on board. If you have any doubts, check the cannonball (test measurement).

Buy a gallon of water at your local grocery store.  As you check out ask them to weigh it.  They have very accurate scales right at the cash register.  Write it down.  Take it home.  Weigh it with your fish scale.  Compare the result with the grocery store.  

 

From then on, adjust your observed weight on your scale by the error between the grocery store and your scale.  My Rapala weighs one ounce light, so I just add a ounce to what it tells me the fish weighs.  

I usually have a pretty good idea about what a fish should weigh, so unless it's way off from that I'm not terribly worried.