Adjusting Blower Speed for Central AC
When I was still doing residential work the normal temperature drop across the Coil was between 15F.  and 20F.  A temp difference above 20F meant that more air flow was needed and a temp below 15F meant that there was too much air.  Unless things have changed recently or you have a special A/C system you can use these temp drops as a guide line. Just buy a temperature thermometer or a probe type and measure the air temp difference across the coil.  my 2 cents