As a starting point, I want the 3 pins on an attachment to line up in a vertical plane before I start making adjustments. Based on that, I'd want a top link that puts the top link pin in that position when it is in the middle of its adjustment range. That lets me tilt the implement forward or back and still have plenty of threads engaged.
It really depends on your particular hitch geometry, so I'd say go out to your tractor with a tape measure and approximate what length link would set your implement vertical, then make sure the link you buy lets you adjust a few inches longer than that.