How do I prune a butterfly bush?
How far back should I prune? I want it to grow to about 6-8 ft.
Have you ever pruned it before? If so, you have an idea of how much it grows after you prune it. You also want it to be full and have a nice shape. This article will tell you the basics : https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-pruning-how-to-prune-a-butterfly-bush.htm
Don't be afraid to experiment until you get the look you want.