I've created several tattoos for people (including myself) which, in order to line up correctly, requires perfect measurements around the body part. 

I incorporate knot styles and line weight according to the client preference, then they take the finished design to a tattooist who has a transfer creator to ensure the linework is copied exactly and still fits perfectly.

The above and below photo are of a bicep, shown first at the inner elbow, and below from the inner arm.

This rather blurry photo is unfortunately the only one I have of this ankle design, which the client wanted to incorporate a woven X and O onto the front of her ankle, along with ivy leaves growing out of the wraparound knotwork that meets at the back of the ankle.