When setting up a grid of trotting poles to a cross rail, the distance from the last trotting pole to the cross rail should be:

The distance from the last trotting pole to the cross rail being double the distance between the poles allows for a smooth transition for the horse as it approaches the jump. The purpose of the grid is to help the horse maintain a consistent rhythm and balance while trotting towards the cross rail. When the distance is set to this proportion, it encourages the horse to naturally lift its front end and prepare for the jump, minimizing the chances of rushing or taking off too early.

This setup is particularly effective in helping both horse and rider develop an understanding of proper take-off distances, as the horse has the time to adjust its stride without the poles being too close, which could lead to confusion or misjudgment. It creates a flowing and rhythmic exercise, which is beneficial in developing the horse's jumping technique.