Difficult to answer, my experience has only been with long stroke motors at lower (~9.5-10.5K) rpm.

I've found on them that setting the length sort of depends on the pitch and how you fly, but I usually set the pipe to a length where I get a smooth transition from idle to full power (over about 5-10 seconds) on the ground. Set a bit short and it will tend to struggle to get up to top rpm because it'll be a bit rich, then lean bit out as it gets on the pipe. Set a bit long and they seem a bit difficult in the midrange in the air as it tries to get onto the pipe at part throttle especially if it's a clean airframe and running less than 11 inches of pitch.

If the pipe's set shorter I need to flog it a bit ie. get on the power early, let the rpm build then pull the verticals because if I get on the power late and it doesn't pick up that couple of extra rpm it falls off the pipe in the climb. With the pipe set about right you can feed in the power as you pull vertical and it just keep pulling but just a little less strongly.

Buy two headers that way you can keep cutting one until it's obviously too short for the prop you've settled on.