Running pump slower is fine. It is not linear but close to amount slowed. Only way to know exact would be flow test. If dropping 10% on 7 gpm pump you would loose about .7 gpm. Never slowed one down but have sped them up and they followed that so only stands to reason it will work same.You could just run a pump sizer which basically is just a jetted line to bypass x amount of fuel. Would be easy to adjust and changes are just a jet,spring or shim in a poppet.