would a kitchen knife work?

Not thin enough, .15mm is very thin. At or less than the thickness of paper.

The issue is that the bed to nozzle clearance dictates your first layer thickness and squish. The first layer should be at max half the thickness of your desired layer thickness. I use .15mm so I am good for a .20 or .30mm layer thickness. If I ever went below .20 I would then change to a .05mm bed to nozzle clearance.

what do you mean by leaf?

Lets call it a blade then. Do a Google search for feeler gauge if you are undure what I mean. They resemble a pocket knife of sorts with many blades of specific thicknesses.

Hi i recently flashed my Da Vinci 1.0a to Repetier and I was wondering how to properly calibrate it easily. I tried using the manual mode and i was able to put a piece of paper between the extruder and the bed. Is the paper supposed to be barely able to touch the extruder and easily slide or does it have to be touching it and moving the paper makes a rough sound or something like that? thanks

You should feel a slight drag when you move the paper under the extruder. If you have access to or can or can purchase feeler gauges the .15mm leaf works much better.