You pull it tought enough so that there are no wrinkles. As for how agressive is the shrink, these are the elevator halves for my Jungmeister. The black is 1/8 inch diameter carbon fiber tube. I used taughtening nitrate dope for the sealer/shrinker and it pulled the 1/8 tube out of straight by about 1/32 inch on each side.

Contrary to common belief, nitrate continues to shrink for weeks after dry enough to touch, and will eventually pull out any sag there is.