One of the biggest things that I know have helped many riders with their spots is counting with the stride! Just count with the horses canter beat (one two, one two) then as you approach the jump you will be able to determine how off you are so then you can't up to with 1,2,3 and on 3 the horse should jump. I know this sounds compicated but it's really not, abd it does help! Give it a try! I hope I helped!
Keep a steady rhythm throughout the course, and you should hit all the jumps in stride. Of course you're still going to have to turn at the correct time and count your strides, but keeping a consistent pace is crucial.