Nice work.
What would be possibly more accurate would be to make a jig out of one of the large sealing caps.
They are like this...
http://www.cobrainks.com/product_images/d/335/cli-8bk__82551_thumb.jpg
I say possibly more accurate as the paper template depends on the ability of the user to stick it down accurately each time.
If the end of the clip was accurate drilled then a cart would be held securely in the same position each time, and the whole thing held easily while drilling the fill hole.
Me, I'm just too lazy to do it! I found by studying several picture of refilling on the net I could make a small starter indent in the exact place on the cart (assisted by an eyeglass), then drill slowly by hand with a suitable hand drill.
I think your template will be very useful though to those not so certain of their abilities to accurate position the drill hole using their eyes alone.
One suggestion for your template. If you put a line at the top with an accurate calibration size, i.e. 50mm long, then others can check by measuring the line that their printer has produced the template at 1:1 ratio.
If you look at the following page and click on Stephenson protractor you can see an example of using the calibration line
http://www.vinylengine.com/cartridge-alignment-protractors.shtml