Testing and measuring duct losses:

Ducting losses are hard (to measure) because they rely on knowing your duct material coefficients. You can measure the losses in the duct after it is built and running, if that would help. You could measure a test section to calibrate that material, then extrapolate. Here's how:

Take a known fan (or the fan you will be using) and blow it into a plenum that has a heat source and some of your sample duct mounted to it. To do this, you need a trouble-light or other low wattage known source and a cardboard box to put it in, then mount the fan on the box and stick the duct on the other side.

Calibrate the box by measuring temps without the ducting, then compute CFM. Add the duct, measure the new temp, compute the new CFM. The difference is duct loss. Basically, use temperature and wattage to measure airflow and compute duct loss.

If you have an existing room, just measure inlet and exhaust temps, add up the watts, and then compute effective airflow. I just did this for my box and it's pretty much dead on. I think it varies by about +/- 0.2 DegF for 150 watts and two computer fans.

Once you have the value for your ducts, you can estimate loss by adding up the length. We would have to come up with adjustment for going around corners.

I once saw a Mech Eng. book that had different shapes of pipe listed (Tee, 45 Deg bends, 4-way branches, Y-branches, etc) and then gave an equivalent length of straight ducting they add for flow resistance.