Does my HVAC guy know what he is doing?

First off im new here and thanks for having me. Alittle backstory… I am a heavy commercial, heavy industrial union electrician. I am having a guy that works for a contractor that’s on a lot of jobs we do redue my my ducting in my house. he has it all drawn out and im starting to question the sizing of the ducts he planning to install. ive seen lots of his work and its clean but im questioning his ability to properly size the system cause he is used to working off engineered plans. I have a 80k btu 96% furnace with two stage variable speed fan 1600 cfm, 1100 sq feet of heated area between 5 drops. existing 32X18 return air grill he plans installing a 14” round flex. off the supply plenum he plans to run an 8” round flex that will branch to 2 existing 6” registers one room being 230 sq feet and the other being 140 sq feet. another 8” run off the plenum feeding two existing 6” registers with each room being 300 sq feet. and lastly a 6” off the plenum feeding an existing 6” register 3 feet away on a 140 sq foot room.

now I have no problem admitting im wrong but before I make an ass out of myself I would like your guys opinion. here is what I have come up with… return air round flex needs to be 18”. duct from plenum before it splits to 6” and terminates at existing registers that feeds 230 and 140 sq foot rooms needs to be 10”. the duct from the plenum feeding the two 300 sq ft room should be a 12” before its spits off into 6” and terminates on the registers. also should I be asking him to install dampers on all three ducts that leave the supply plenum

am I stupid and overthinking this or is this guy alittle slow.

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