grow room set up fan speed

It'll depend on too many variables. But, you can see how much negative pressure there is at various levels to figure out a 'max' you want to go to and a minimum you don't want it falling below. number of bends in ducts, filters, and all other things that impact static pressure along with specs of your fan make this diffiuclt to give a one-size-fits-all answer. But, easy trial and error. Take some notes on min max, then use that when you program your climate control options. you want to maintain some minimum air exchange, then a max for heat or scales up with heat, if that's an option... (e.g. a curve like for a GPU cooling).