Jeff:

Your on-screen measuring device must need to be calibrated. I used 2" tape. http://www.cooknwithgas.com/Sailing/Blue_Hulls_Tape.JPG

I recommend using a heavier tape like 3M masking tape. The "duck" brand works ok if you don't leave it on too long and it's cheap it tears a lot when removing the tape.

Have a good day,

Scott Laughlin
Omaha, Nebraska
www.cooknwithgas.com