To get a good idea of where to install the sprinkler head I would suggest making up a few stakes with string attached whose length is about a couple feet shorter than the range of the individual sprinkler heads. use this to position the location of the sprinkler heads. This will help ensure you get total head to head coverage, especially should the water pressure drop a bit.
Also, I would suggest cutting up a 3" dia PVC pipe into 5" lengths to install around each sprinkler head. Surrounding the heads with these sleeves will prevent grass from growing into/around the head over time and provide some protection should you drive over the sprinkler heads with your lawn mower, especially if they don't sit down completely when the water is turned off.
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