I designed and installed the duct work before I had a copy of HVAC calc. I expected an energy efficient design, but not that low. But, I did install dampers in the duct work so that I have manual zoning control and can increase the throw at some registers by shutting others off.
Calculating actual numbers for the total static pressure seemed like a real waste of time. On the supply side, they had to use enough flex duct to connect up to all of my hard metal duct, so I know there is no problem there. The biggest restriction in the system is the 14" flex duct for the return air.
The variable speed blower means that a very low static pressure will just lower the energy requred by the blower.
I did include the fresh air intake on my HVAC calculations.
Besides the 2 ton central system heat pump, I also have a Fujitsu mini-split inverter driven system with variable capacity between 5,500 BTUH and 19,000 BTUH.
So, the total rated cooling capacity for the house is variable between 5,500 BTUH and 43,000 BTUH for a house that HVAC calc says needs 16,137 BTUH of cooling. I did not see any point in looking at derating factors.
I do have radiant barrier decking on my house. I VERY seriously looked at going with a hot roof and it was part of the plans for many months. I really wanted it to work for several reasons, but it just did not work for our design.
My wife and I spent a lot of time looking at new houses while designing ours. We wanted to make sure that we spent our dollars in ways that added real value to our investment. The ONLY place we ever saw sheet metal duct work was in modular housing where the duct work was under the floor and sheet metal was required by code.
Houses that cost more than 10 times the amount we are spending on ours still had all flex duct. The only way that we could fit the sheet metal duct into our budget was for me to DIY.