I'm not an expert..but I play one on TV

No seriously, I work in the air movement business and have spoken to a
few of the people that I work with. They say a big factor may byethe
number of corners you have introduced in the line. You can expect a
decrease of 8% for every 90 degree turn you introduce. This includes
all connectors. Hope this helps. Fogive if I'm stating the obvious.
Sean