I usually am on at about 5 strides out. Now im not saying that I SEE the spot from 5 strides out, I see the spot much earlier than that, but I have an adjustment time where I rate my horse and that adjustment times ends when im from 3 to 5 strides out. What I mean by that is I see where im going to take off from way back, I have that time to fix if I think well chip in or be long, but I STOP my adjusting usually 5 strides from the fence, if its tight then I stop within 3 strides of the fence so that we can focus on the jump and I don't throw my horse off too much. So when im 5 strides out I know exactly where im going to take off, I've planned for it, and I don't try to change it from there. If I judged wrong then my horse saves my butt and we ride it the best we can :) but after 11yrs of jumping its usually pretty good. Since your starting, what I suggest is to set up exact lines, know what theyre set at and count your strides between. That way you get a feel for it. Then advance to jumping a single fence and try to count from 3 strides out. 3-2-1-jump. Say it out loud to help. Once that goes well move on to counting from 5 strides out etc. Then follow what Strange said about adjusting the stride in between. Id wait to do this until you can count out properly set strides. But move on to making a 4 stride a 3 or 5 or 6 stride and counting and feeling the pace. Itll really help! But my rule of thumb is that once your within 5 strides (occasionally 3, depends on the turn and jump etc )of the fence you don't change your ride. I go more on feel like some others have said, but not mistake that for not finding a stride ahead of time! I may not literally COUNT strides, but in order to have a good feel and know where your going to be comes from counting to figure out the stride of your horse and where it puts you. Once youve gone through that training then you will be able to feel the pace of your horse and know what amount of ground your going to cover. They go hand in hand. Another thing to practice is riding along a straight arena and counting strides down the line without jumps at all, then set up random poles and just try riding to them and finding an even rhythm as far back as possible.