Focus more on stretching if going simply for length. Jelqing is much more of a girth exercise than a length exercise, however it is also good for overall penis health which is why it is included. Jelq around 25% or 30% for length. That way a decent stretch is achieved while maintaining decent expansion. Additionally the blood will accumulate the most at the end of the CCs beneath the glans and cause the ends to stretch the most.
Although don’t take my word for the absolute truth, perhaps a vet would have better knowledge on the subject of jelqing for length.