Measuring the same glider on different days in addition to the symmetry results have given me + - 2mm variance. Generally a tolerance better than factory for line length. Plenty of accuracy to return the glider AOA to the factory trim or determining any issues.

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Math checks out to be about +-2-3mm as mentioned by mountainrideaviation. The leica disto d1 has an accuracy of +- 2mm and this is constant up to the maximum range of the device. I dont know the dimensions of the business card used, but lets assume a worse case scenario of a credit card turned sideways and measuring the distance to the very end of it. I'm also assuming the line length is 7.2m (similar to an artik 6). The measured length is the hypotenuse of a triangle with the line length (actual length) being one of the sides and the offset of our measurement (how far to each side of the card we measure) being the other. So a bit of math and C = sqrt(7200mm^2 + (53.92mm/2)^2) = 0.050mm. So even doing it this way without the linear rail, as long as you hit the card your measurements will be within +-2mm + up to 0.05mm. BGD specifies for their gliders that a line should be within +-10mm to be acceptable