To determine pressure loss, you've just got to look that up in pressure loss charts. But to do even that, you've got to know about how much water should be flowing through the poly. Will it be a single rotor where flows can easily exceed 5GPM, or will it feed a single spray head where flows are likely to be on the order of 1GPM or less?
Here's what I can tell you based on what you've provided so far...
Using 1/2" poly (or 1/2" PVC) you are going to limit your flow to about 5GPM through that 1/2" pipe. If the head(s) at the end of the poly need more than 5GPM, then you will be violating the '7ft/sec speed limit' of the water flow.
At 5GPM, you are going to loose about 1psi for every ten (10) feet (i.e. your 15' of poly would have about 1.5psi loss).
At 1GPM, you are going to loose about 0.5psi for every hundred (100) feet (i.e negligable for lengths of 20' or less).
