I'm tying quilts for a local woman too. She said when she entered it in the fair last year they put a note on the back of her tag. They said knots should be no more than 4" apart. I told her that was pretty much the rule. I just go by my hand. It's easier than getting out a ruler.
The knots are sewed on my quilt I just need to cut the strings between and actually tie the knots. Lion's barter night is 11th so it will be done by then.