I'm severely farsighted with a lazy eye (Hooray. No lasik can help me!
), and I know what you mean. I get headaches and blurry vision if I knit for too long. I try to take a break every hour or so, and I also try knitting with my needles at different distances from my eyes. So I'll knit closer to my face for a few minutes, then farther from my face for a few minutes, you get the idea. Even just a few inches difference seems to help. So maybe just take your normal, comfortable knitting position and add or subtract a couple of inches from that periodically.