Hello Mikey,
The Nozzle Flow Rate chart that is is Delco's Wats Blaster shows that a 1.4 gpm flow at 100 psi you need to use a #9 tip. You would order a 1509 or a 2509 nozzle to get the flow and spray fan you are looking for.
I don't know an easy way to do the math, I always use the chart that Delco has on page 10. I have made copies of it and put them on our trucks.
If you don't receive the Delco publication, let me know your fax number and I can fax you a copy of the chart.
Dave Olson