A loop will not change the mechanical draw length of the BOW, as in the power stroke remains the same. It will however change the perceived draw length, or YOUR draw length because now the distance from your bow hand to your release hand at full draw has increased. It will also change where and how you anchor and the position in which the bow is pointed at full draw.

Every set up is different. I have several bows and every one of them has a slightly different draw length in order to feel "just" right. Even the type of release makes a difference. I suggest when you buy a bow you take your release, or if you are buying a new one you pick one out first then decide if you want to use a loop or not. After that have the bow adjusted to fit you then start tuning it.

You can use a head designed for a loop though that has a shorter head and things will equal out some.

Paul