Since the adjuster screws have a 20 thread per inch pitch, and the rocker arm has approximately a 1.5:1 ratio, 1/4 turn of the adjusting screw gives a clearance of about .020" at the valve stem. That's "close enough" to do a warmup run without an excessive amount of noise, while still giving the valve stems a little running clearance. That's what Dad used to call a SWAG (Scientific Wild-A** guess!)
Jerry
