Oversize finished basement return, pull air from floor above

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I’m currently reworking my HVAC for finishing my basement. The house is ICF and well sealed/insulated. I have a single unit for the whole house. Manual J calcs show my existing unit is adequate for the house including the basement addition. I’m relocating the unit from one part of the basement to another. The physical routing for the existing return limits it’s size to slightly less than manual D would say. My main floor has a loft though that doesn’t have a return. My thought was to put a 12” duct and pull a return from there to a register/registers in the ceiling of the finished basement then oversize the return down there to handle the difference. I see a benefit of pulling this warmer air through the basement to bring its temp up to the sane as the rest of the house. I experimented with a sun room zone return doing this as I had to cut the duct to frame a wall. That one return 100cfm boosted the basement up a couple degrees. We run the house at 74 and the basement sits around 68. I can put a booster fan in the 12” line to the basement if needed. There is no good routing to get the return directly to the unit. The stairs to the first floor has a door so I’m thinking that jump duct would be a path of least resistance? Any thoughts or critiques to this? I’m hoping this would keeo the basement zone to where someone doesn’t want yo run the heat in the summer….
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