So, I've not considered a well source before, but I would think irrigation design would go like this:
- Figure out how many how heads you'd like to run at once. For each head, look up the design pressure requirement and flow at that pressure.
- If the well pressure switch is set at 60/80, then use 60 psi as your design available pressure. [Not sure about this, not so familiar with wells, but it should be conservative.]
- Verify your well and pump can provide the required flow at 60 psi indefinitely. [Otherwise, reduce heads/flow.]
- The difference between 60 psi and the sprinkler head design pressure is your available pressure "budget". That budget goes to (a) elevation difference between the heads and the pressure tank and (b) frictional losses on the piping from the pressure tank to the heads. If your actual frictional losses exceed your budget, then either reduce heads/flow, or enlarge pipes.
Your procedure with the hose bibb lets you measure the frictional loss your existing piping is causing, with the caveat that the hose bibb and hose are causing some pressure loss that a permanently installed system wouldn't have (particularly the hose bibb, I would think, they have pretty small internal orifices). Not sure how big an effect this is.
I assume you are using a wye at a spigot, with the pressure gauge on one side and the test sprinkler on the other side? Then the data you really want to capture, at some instant in time, is (A) the flow rate out of the sprinkler (B) the pressure at the spigot and (C ) the pressure at the well tank. The difference C - B is the pressure drop of your piping to the spigot (+/- elevation correction) and A vs (corrected) C - B characterizes your existing piping. The bigger the pipes, the smaller the (corrected) C - B for a given A.
[If it is difficult to capture both C and B simultaneously, then there are way to infer C. For example, if you note B and turn off the spigot really quickly and then immediately note the pressure reading, that is approximately C (along with the correction for elevation difference, which is good). I say approximately because if the well pump is running, the gauge should be moving up to 80 psi (+/- elevation correction), so it will have changed somewhat while you were turning the spigot off.]
BTW, drawing water at one hose bibb while measuring pressure at another isn't a useful procedure for a well fed system. For a city water fed system, measuring at a hose bibb right where the water lateral comes in plays the role of measuring at the pressure tank in your system.
[Edit: second everything Reach4 said about filters, I hadn't considered those.]
Cheers, Wayne