Try laying the tile down flat in front of you EXACTLY in the orientation it will occupy upon installation (finish side down) and then when you transfer your measurements from the inside vertical piece of the grid they will fall right where they should on the back of the tile.
Measure to the pipe and add half of the diameter (trying to eyeball the fattest part of the round outside takes some practice, so just measure TO the pipe and add 1/2 of the outside diameter of the pipe) Do this from two perpendicular sides. Poke an ice pick or an awl through where the two lines cross. (That will be the center of the hole.)
Turn the piece over and carefully cut a circle out with a slim, sharp kitchen knife. The hole should be just the size of the pipe, or maybe an eighth of an inch larger.
Of course, to do this right, the water has to be shut off and the heads all removed first. (NOTIFY YOUR FIRE MARSHAL FIRST !!!) It's REALLY hard to get a perfect cut when trying to encompass the entire installed sprinkler head. REALLY hard! I wouldn't even suggest trying. Getting past that head hanging down is tough. And even trying to is just punishing yourself needlessly.
THEN you will usually have the hassle of getting the piece of tile to fit the pipe AND the grid all at the same time. No where near as hard as getting over the whole sprinkler head, but still a challenge. This part just takes patience. Often you will find loosening a few wires on the grid helps so you can lower the grid a little...... but remember to put those wires right back up in the proper original elevation.
Sometimes you get lucky, and you can lift the pipes a half inch or so. But not often.