Anybody try this? Find the linear density of the string in question by cutting off the ends of the (old, unused) string just before the thread and weighing it on a small scale. The linear density is the weight divided by the length of the sample. You can then use the string equation to calculate the tension at a given pitch and string length. I bet this is how the manufacturers do it too, since it is easier and probably more accurate than tighting up a string with one end connected to a big scale.