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This cutter features 2 depths. The measurements are from the center of the bore to the bottom of the window's depth. For best use I HIGHLY recommend dripping silicone oil on the cutting blade and in the slit that is being cut in the material. This prevents static friction from grabbing the rubber and pulling it with the knife, preventing uneven cuts.
I measured ZCI hop windows to be 2.46mm deep, so the 2.5mm side of this should get really close. The 2.6mm side can also be used to leave more wiggle room to sand father more precisely in the case the cutting is giving the end user trouble.