As the head heats, valve clearance increases. This is because the head, being aluminum, expands more than the valve does, moving the cam up away from the valve seat. The valves heat more quickly, and run hotter, but at full operating temps, the clearance is slightly greater than when cold Your intake valve clearance is .10-.15mm, not .15mm. Trimming shims is not a good approach. There is no possibility that you will get both surfaces parallel to each other. Just buy one. What size shim do you currently have?