It depends on the stride of the horse, but for trot cavaletti, start at 4 1/2 feet between poles, with a 9 foot gap between the last pole and the crossrail, then adjust as needed for the horse's stride.
Also getting in deep/short indicates carefullness and/or a general lack of confidence, so lots of cavaletti and grids are in order here.
How adjustable is the horse's stride? Doesn't matter how good your eye is if you can't get the horse to adjust to meet what you see.
And finally, a pet peeve of mine, sorry. I believe strongly it's a BIG mistake to teach a green horse to rely on the rider's eye. That's called professional ride only, unsafe with a novice rider or difficult to sell in my world. Much, much better to give the horse the tools to find their own distances and negotiate fences safely from any spot.
When schooling a green horse, a certain period of time should be spent "riding like a beginner" - aproaching fences in two point on loose reins and encouraging them forward.