No strap on main awning. How do I know the length I need?

Wish when we got our first RV with an awning someone would have told me to get these So simple to use after retracting the awning and it is in travel position simply wrap one around each side arm and slide it up as high as you can reach. If you just forgot to lock them in to travel position or they come unlatched the awning will not unfold going down the highway. To this day I do not know if ours came unlatched or I forgot to latch it either way since I did not have these it cost a new awning.