<<If someone does a manual d calculation on your existing duct system...how are you going to know they did it right ???
Do this...get in an hvac contractor. Get them to make a detailed drawing of your complete system..making note of the boots, the type of takeoffs used, the way the fittings are made...90 ells with straight throats , whether or not turning vanes are used, radius ells or square ells etc.... get an exact drawing with sizes of your complete duct system. make copies for yourself. Then decide on a piece of equipemnt....it must be the same for all calculations.
Then...give a copy of your drawings along with equipment info and any added accessories and have three contractors do a manual d on the set up.....
then come back here and post your findings
Isn't this what a lot of residential owner's need --- a skilled HVAC specialist to design what is needed, and then aid them in getting good competetive bids and post inspecting the work once it has been done?
Do you suppose people would pay for that service?