Precise Magnet Catch Placement

I'm using a magnet catch to hold the side doors closed.  A rare-earth magnet sits in a little metal cup, which amplifies its magnetic force, and the door gets a little metal washer.  The only tricky part is locating exactly where the washer needs to be so that it mates with the magnet when the door is closed.

Magnet-in-a-cup

Magnet looking for Washer, must be attractive and enjoy dark places.

Sure, I could get it in the ballpark, but I'm fussy.  I want it spot on.  My first idea was to put the washer onto the magnet and then paint the other side of the washer with wet red nail polish.  Close the door, thus transferring the red paint onto a specific location on the inside of the door.  

That was the plan, actually, I was just waiting for my wife to get me some of her nail polish, but by the time she came back downstairs I had stumbled onto a much simpler, and better solution, and had the entire thing assembled.  

1) Put the screw into the washer and flip it around and slide it onto the magnet.

2) Close the door and press it onto the point of the screw.

3) Drill where the dent is.   DON'T DRILL THROUGH YOUR DOOR!  OMG, I've done this too many times...
 

You're done.  Enjoy the satisfying click as the washer is perfectly seated onto the magnet.  Open and close the door several times in a row just to hear it and feel the solidity.  Mmmm, quality.