On glow engine tuning it is wise to cut off a few pieces of different length, and please do not discard them. They can be inserted into the silicone tube coupler again to lengthen the header if so needed.
open space between coupler inserts should never be more than 1/4" (6mm)
In gas engines, a long teflon coupler can be used like a slide trombone, because the coupler has very little flexibility. In glow engines that use silicone couplers this is not possible. The silicone is too flexible to allow lengthening without inserting some piece of rigid tubing.
For best tuning, you do need the lengthening option as well, so you get best in flight results after trial and error.