Jennifer Crank says:

Knowing that our dogs naturally run parallel to us, convergence is always what I would consider a trained skills. One of the most common example of convergence is a serpentine. We often hear of people having trouble with the dog coming in over the center jump. In many cases, a handler may proactively or reactively rotate towards their dog with an outside arm to help the dog converge.

For training the skill, I start by teaching stationary recalls (collection recall, slice recall, etc) and then progress through the training of adding motion. This teaches the dog what is expected when motion is added and places value on the decision to converge over the jump. When applying it to a course I will almost always use my dogs name for convergence. That is what I’d consider my cue. While my seasoned dogs are likely to do so anyway, it helps remind them that I want them to follow more than my motion.

In this scenario, if you decelerate too much/long at 18 you might not being ahead for the push from 19-20. That is one important part to convergence – you must be ahead to do it. If you were to decelerate at 18 and fall being going to 19, then a rear cross on the flat might be a nice alternative to make sure the dog turns to 20.