Be patient. It'll come with practice.
Just like learning to dribble a soccer/football, or ride a bycicle, or swim (not to mention different swimming strokes), most of it comes down to muscle memory.
After some time spent practicing (with the same harp, I might add), you'll get to the point where you
know exactly where you are on the harp. Mostly because you can feel it. You know what it feels like (here's where the muscle memory comes in) to move the harp x-many holes one way or the other.
Be patient. You'll get it. It just takes a little practice.