The photo below is of an installed length of aluminum semi-rigid duct which is located in the space between the roof and the suspended ceiling (with ceiling tiles). It branches off from the insulated rigid trunk of heating ductwork also located up that that area. From that main rigid branch the other end of the flex is connected to the ceiling register (or diffuser whichever it's typically called) installed into the suspended ceiling approximately a distance of only 4-5 feet. However, the original installer chose to use an excessively long length of this flex duct, probably a good 12-14 feet, to connect between the two points (the photograph clearly shows the indirect routing, how it snakes and curls up and around and then back down). My plan is to cut the flex shorter and reinstall between the two points. My inquiry here is what's a good method to measure the distance between the two offset points so that when I cut the flex I don't cut it too short but yet don't end up with it still unnecessarily longer than it needs to be, so the expandable "accordian" aspect of the flex material is stretched out to it's maximum reasonable/proper more ideal intended installation expansion.

 

Remove one end and run it the way you want it. Pull it tight cut it off then attach the one end back.

 

I was thinking/hoping to remove the entire length from the space up there, take it down to where it's not so cramped and awkward to work, and cut it shorter to appropriate length then take it up and reinstall. So while it was removed I was thinking isn't there a method I might be able to relatively accurately measure the length between the two open connection points, so I'd know how long to cut it.