i asked what the guidlines were when i went through my medical for nats back in 2002. The limits on the class 1 medical was +-5 for sight , although exceptions on that did happen. Nats however set there eye sight test at +-3 of which i was +2.75 in both. On asking why it was set at this, it was based on the fact that students at the time were aged 28 or below and factored in the likelihood of deterioration through age. However older students were assessed slightly differently, as age was factored in.