The Geo-Disc is pretty useless for a significant percentage of tonearms out there ... including the Linn.
Linn's own protractor disc, as part of the Speedchecker, works very well for their arms -- allowing you to verify the alignment on both the null-point grid and the overhang arc, both optimized for your arm geometry. IIRC the pricing isn't too outrageous ... the complete Speedchecker isn't too much more than a high-end protractor like the Mint, and you get highly accurate speed calibration as well.
Another nice tool is Linn's Kinky, which fits in the arm collar and verifies the exact spindle-to-collar alignment. It can reveal such issues as the armboard out of alignment, or a bent subchassis. They're expensive, but a nice Linn dealer may let you borrow one.