What are the flow rates of the existing zones? Once you have that number, you can try it on a pressure loss table. With well water, however, you can lose the ability to undersize a distant zone, because of pump and pressure switch concerns. You would likely pick a pipe size from the pressure loss chart that would lose only a fraction of 1 psi per hundred feet. That would eliminate any worries about the distant zone not having enough pressure, or its not being compatible with the existing pump control settings.