You will get many that swear by that method.
I have found it to beless than accurate for all.There are many pros that shoot longer than the wingspan method would suggest,I shoot longer than the wingspan and I have had a level 4 coach help me with draw length,among other things.
As far as a loop,you will also get some that say the loop changes anchor,not draw length,I disagree with this and I think changing your anchor to accomidate a loop is a mistake(other thanSLIGHTmodifications).I believe you need to make the equipment fit you,not try and make you fit the equipment.
Heck,my draw length changes from release to release.The bow I currently shoot is about a 1/8"-1/4" long on draw with my wrist release but with my handheld and back tension,it is dead on.
I would suggest totry a bow set at 28 1/2" with a loop and see how you like it.My gut feeling is you will like it.
Of course,without seeing you in person,I can't say for sure,just pure speculation from the info you gave.