extending a plenum
I have a reducing extended plenum HVAC system in my basement. In order to clean up the ducting before finishing the basement, I moved a duct from ~ 4 feet from the end of the plenum to about 12" from the end (take off is on the top of the plenum). I know that there aren't supposed to be any ducts within 18" of the end, so I was planning on extending the plenum another foot or so. Is there any issue with doing this? I've seen lists of guidelines that seem to indicate the plenum length is supposed to be some specific ratio to the air handler dimensions.
Is there anything else to know about extending a plenum?