I always set it at the lowest , as in your case 38 . this allows the gap to stay in spec longer as the electrode wears the gap gets bigger
Cheers Slider

there isnt really enough difference in the motor to need a difference in gap, bigger gaps are needed in hi reving hi compression motors cos a small gap starts a small flame which can actually get blown out, if the gap is too big the coil wont recover in time to fire on the next cycle, and a poor spark will result, more obvious in dual fire systems. 0.040" is good.