Well since you are 37 I wouldn't worry about it If your maual doesn't tell you what the correct head clearence is for nitro percentage or how many shims to add then I call the manufacturer.And if I can't get ahold of the manufacturer I ask the guys that are running the motor or have had the motor.Most engines come shimmed for 16% and 20% nitro.You have to measure the head button clearence to get the shims exact.Or you can add shims till it doesn't detonate which will leave pitting in the head button and piston.I have not tried this but most people I believe take a piece of solder and stick it in the glow plug hole and turn the crank over to tdc then pull the solder out and measure the compressed peice of solder to determine the exact amount of shims needed.Best bet is to call the manufacturer as they are the ones that built the engine and ask them.