I think the machine is fine. The problem is that you should only use the large weight for your ref tension of 55 lbs. If you use both weights you'll be off about 5-6 lbs. Placing the yardstick graduated number at the bottom of the weight is correct.If you look at the yardstick you'll see pictures of the small, large, and combined weights to the left of the 12-23, 20-68, and 66-80 markings. So if you only use the large weight and line up the bottom with 55 you should pull 55 lbs.
MS200TT, Babolat Star 5, USRSA Certified Stringer