I think Zyrusse's illustration is perfect - the first throw you make creates an imaginary line, and you need to figure out where along that line to dig. The best way to do that is to go along 1400-1600 like you said, but then turn 90 degrees and walk 20-30 blocks before throwing your next eye. The line that the second eye creates will intersect with the first one, and that "x" marks the spot.

Sometimes the second eye basically goes in almost the exact same direction as the first, so you know you have to go a ways yet, then basically repeat.

Interesting and innovative strat idea, but I'm gonna guess this wouldn't be allowed; it'd be kinda like unofficially using using x-ray.