The Set CFM Total In a Duct command is used to set the total airflow in a duct. The airflow is distributed to all of the diffusers connected to the selected duct.
To set the total airflow of diffusers connected to a single duct, go to
Ribbon: DM HVAC->Diffusers-> 
Set CFM Total In a Duct
Pulldown Menu: DM HVAC->Set CFM Total->In a Duct
You will be prompted to select the duct for which the total airflow for all connected diffusers is to be set.
Select duct or label:
The Adjust Duct CFM Total dialog box will appear.
Adjust Duct CFM Total Dialog Box
Old CFM: The current airflow through the duct.
New CFM: The new airflow for the duct.
See the CFM Setting Adjustment Methods section for more information about the Average CFM in Diffusers and Prorate CFM in Diffusers fields.
Setting the Total Airflow in a Duct
When the values in the dialog box are set correctly, press the OK button. The new CFM value will be set in the diffusers connected to the duct.