I was just testing the tension on a used machine I just bought, a Tenex/Gamma 6000. I find that on tensions 60-70 lbs, the calibrator shows the tension is accurate. But if I test for lower tensions, 30-40 lbs, it's out by as much as 5 lbs. What is this indicating about the tension spring? Is this anything to be concerned about? I'm mainly string tennis racquets, so it will be in the high 50's to low 70 lbs, so does that mean I'll just have to compensate if I'm stringing racquets that require lower tension? Thanks.