When you select the orifice, its Property Grid will be displayed, shown below. Simply vary the diameter until you get the desired flow rate through the pipeline.
Figure 1. Property Grid for a Balancing Orifice device.
By taking advantage of calculate mode, the system automatically recalculates the pipeline's flow rate each time a change is made to orifice plate size. You will need to hit "enter" or click your mouse on the FLO-Sheet to commit the diameter change and see the new flow rate results displayed. You should be able to arrive at the desired flow rate in a few iterations of the orifice diameter. Figure 1 shows the orifice diameter in the Property Grid being adjusted to 1.25 inches to achieve the 294.8 gpm flow rate through the recirculation pipeline.
Figure 2. Recirculation orifice that has been sized with PIPE-FLO® Professional.