Remove all the final fixings. Expose the pipes.
Offer up the tile just under the pipe. Mark the centre points at the edge of the tile
Offer the tile up to the side of the pipe. Mark the side of the tile.
Draw lines into the tile to find the centre points.
Use the drill plate and a pencil to mark the hole size and location on the tile.
Drill the holes into the tile.
The results of drilling will be a perfect hole in the slate tile. Offer it to the wall and over the pipework.
Most fittings will cap off and hide the hole (shame..) so you may not have to be millimeter perfect. Also if you are slightly off with accuracy then you can use the sides of the drills to grind out to the correct position. But I am sure you are accurate so will not need to....
Slate is softer than granite and porcelain so easier and quicker to drill holes in.
If you have more questions of course just call me..
Richard