What are the composition guidelines to be followed in line art of type Mehndi designs?
I am also from India and i know about mehndi, while creating any mehndi look on hand, you need to have a Top down approach, the creation is started from the top so as to not disturb the wet mehndi you already applied to the hand and you have space at the bottom to rest your hand and create a design. 

While on paper, there are no rules, it's basically doodling, you can get as creative as you want. The creativity applies to both hand and paper.Also you need to alter your designs based on where you want them, for a hand the diagram you mentioned might not work because a hands shape and size also need to be taken into account. I hope this helps because your question is a little ambiguous.

mehndi is a form of doodle art, some examples ad rules can be found at:


http://drawsketch.about.com/od/doodles/a/Begin-Doodling.htm
https://www.pinterest.com/pin/185773553351220298/
http://www.sparkletart.com/rsr/2013/08/doodling-for-beginners.html
https://www.pinterest.com/elixaa/henna-on-paper/
https://www.youtube.com/watch?v=-1ub7RR2GNY
http://www.lifechilli.com/draw-mehndi-designs-step-step/ (for hand, just as a refrence)