All you do is use two or three blades and add them up, easiest way that's how l always done it, of course you have to be careful with silicone you only need a tiny amount it can block oil ways and cause oil starvation if it gets in engine. It won't hurt to ride at 20 it's when they are to tight damage will occur it will burn out the valve. No way will your engine blow if the clearance is just a tiny bit to big, remember if they are to tight that is what will do the damage an expensive repair, the head has to come off new valves and regrind if you are very lucky, at worst seats will have to be recut reground.