With the throat strap you should be able to easily get to fingers under there. With the belly band, 3 fingers, otherwise it will make him sore. When you have your chest strap on place it do the top of it is at the V in his neck. When you step back and look at his side the harness should create a straight line from front to rear parallel to the ground. The harness works on opposing forces. If a strap holds something up, another will hold it down. The chest strap pulls for forward movement, the breeching opposes this by not letting the cart get too close to his hindquarters. The shafts need to be able to move with the horse's body and not be rigid. Since ponies are notorious for wanting to snatch grass, adjust the check rein so his nose can reach his knees but no farther. Have fun.