At a jumping clinic recently, the chap running it had two key things, Rhythm and counting strides.
He got us to do exercises like putting 2 poles (and progressing to two jumps) a longish distance apart, and getting us to shout out the number of strides between the two poles. Then we tried to shorten or lengthen the stride, aiming for doing the reverse to the total number counted between the poles. I think this really helps you know how far your horse goes when they are in a particular rhythm.
Same with jumping two jumps at a distance-the more you work with a set distance between two things, you can get better at generally assessing distances in relation to your stride length.
At least that was his theory- I'm rubbish at it but planning on doing some of the above work this week.