BTU is not the primary metric. It's duct velocity. You want a target of 2500 - 3000 feet per minute of air velocity in the duct so that dust, fine particulates, and fumes are all captured and removed.
A rule of thumb many people use is 125CFM per square foot of hood opening. So, a 4' x 2' hood (8 sq ft) would need a 1000CFM fan. After some math, an 8" duct provides about 2900 feet/min. and would be a good choice. The challenge here is to find some fans you can purchase and work backwards from there to see which one is best.
Also, don't forget about the make-up air. You need an opening that doesn't restrict incoming air or it will lower the fan's performance.
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Bobcat on 15lpm and Phantom on HVLP
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