I use playing cards because they are of uniform thickness…anything else wouldn’t be allowed in a casino. That said, use a digital micrometer to know and understand the thickness of your substrate, playing card, and tape if you are using tape and CA glue. Drop your Z to the surface you wish to set the z (I almost always use the machine bed) and make fine adjustments so your playing card is just scratched by the cutter. Remove the card and drop the z the exact thickness of the playing card. Set the Z. That’s how I do it and I have a super clean spoil board.