The length to weight conversion tapes are a handy and quick 'weigh' to give you the weight of your fish.
To obtain the most accurate reading from the tape its best that you measure the total length, i.e. measure from the nose of the bass to the end of the flattened down (and closed) tail and also in as straight a line as possible. i.e. not over the curvature of the body . One way is to place the fish onto the tape rather than holding the tape over the fish.
Do not look for 100% accuracy and at times there will be outliers from the norm.
Tapes are available from email@example.com