In my area irrigation is a way of life. There are several fairly large irrigation supply companies that exist not only for agriculture but for homeowners as well. There are designers that work for these companies that will take a scale drawing for a landscape and the basic information on water supply, pressure, etc. and design a system for the homeowner at no cost...the catch is, you must agree to buy all of the pipe, valves, and heads, from them. The good new on that it even that price is usually less than the Big Box Stores and the quality is better. The general idea of what they do is to start with large diameter pipe, perhaps 2", then progressively reduce in size as the lines increase in distance. I would urge you to seek this type of assistance. Even if you have to pay a reasonable fee for the design, it will be worth it.