Furnace install in attic - Duct configuration
A 60,000 BTU 90% furnace.
Temp rise of 40°F needs 1250CFM
For 50°F its 1000CFM
For 60°F its 833CFM
For 70°F its 714CFM
If an 80%, then air flow is
Your 10" round, tapping into the plenum, is about 135' of total equivalent lenth.
Each 8" tap will also have an El of 15' mimimum.
Depending how close together they are, some will have to be added together. So some could be 30' or more EL.
So your supply as you have it shown, will have a longest TEL of about 210'.
Since you don't have any return info. We'll say its only 100' TEL.
Allowing for normal filter, supply registers, return grille, air filter, and A/C coil pressure drops.
Furnace rated ESP .5"-.32"=ASP of .18"
(Trane recomends .2" ESP for heat only applications)
10" round at .06"FR is 320CFM at 590FPM
That would give you a total of 640CFM, or a 70°F temp rise on an 80%, or 78°F on a 90%.
Yes, vanes, or a diverter should be used. And blower speed should be choosen wisely.
Vanes will decrease the EL of the round tap into the plenum by more then ½.
So what is your definition of low velocity.