For a preliminary test, use .15 for a go and .20 for a no go. If the .15 is tight or doesn't fit or the .20 is a loose fit, then you should work out a combination that will tell you the exact gap. Use the difference between the exact gap and .175, add or subtract to/from the current shim size (depending on whether the clearance is too small or too large) to determine what shim you need. By the way, if the shim you need is not a size that is made, go with the next thinner one. Too much valve clearance is better than too little.
