These figures are from Keith's notes.

If you have a 14T pinion the center of the Pletti is 50mm aft of the top of the windshield on the center line.

Gerd, from Minicopter, suggest  moving the center 1.6mm back for each additional tooth.

Once last look...  hoping the cut will go well...  :D

You can use a utility knife.  I used a circular cutter.

Mark the center of the motor. 

This will depend on your pinion.

I have a 16T pinion.  So my center mark was at 53.2mm.

Adjust your cutter to 32mm.

Slowly cut using several passes. 

File down the edges after.

Whew!  ...taa daa!

Side view.

Front view.

Insdie view.

Looks like Gerd's1.6mm per tooth adjustment is perfect.

For smaller pinions I'll simply cut forward.