weight and balance question

FWIW having just completed a W&B for a Grob recently. I'd be more concerned about making sure you have all of the geometry measurements correct including (especially) leveling the glider per the W&B instructions in the manual. A couple CM of error in a dimension (e.g. datum to the tail weighing location) has a lot more impact than 1 lb error in the scale itself. If you have concerns about the scale, it's pretty easy to do some simple calibrations/tests yourself. A plastic 55 gallon drum filled to the reference mark with water (for example) gives you a pretty good indication. Don't forget to adjust for temperature in the specific weight of water.