To do this properly you will need software (RoomEQWizard works fine for "snapshopts", but I prefer TrueRTA as I can see the changes visually as I move things around), and a properly calibrated microphone along with a mic preamp.
If you are taking this project seriously, don't use the RS SPL meter, as it is well known that it is not accurate.
If you are taking this project seriously, don't use the RS SPL meter, as it is well known that it is not accurate.