I'm assuming you are talking about how to figure out the length of ribbon needed. For my bows using 3/8" ribbon that will be placed on the end of a snap clip, I make them 2". I use a "formula" for ribbon length (for any bow, any size ribbon), you may already have one established but mine is: the number of loops + 2.5" x the width of the bow

Basically, if you want to make a 2" bow with 4 loops, the formula used to figure out ribbon length would be:

4+2.5=6.5"

6.5"x2"=13"

13" would be the length of ribbon needed. HTH and makes sense