Is there a formula for figuring draw length? I don't need to be exact, I only want someting close when ordering a bow online for my wife.

Try this link.....

http://www.huntersfriend.com/draw-length-weight.htm

The first and foremost thing is to get measured at a pro shop. Trust me... They will know exactly what the draw length should be. Plus, a certain bow type might require a small change in draw length.

if you want to measure her yourself to get close have her stand left or right hand whichever shes gonna shoot put her bow arm out make a fist and touch the wall then measure from the wall to her mouth youll be very close...........

Wingspan (middle finger to middle finger) in inches divided by 2.5 .

Sorry, I should have looked at the link that ButchA posted above.

I would like to suggestthat after carefully measuring, drop down 1" (or more) to the next shorter draw length. Reason(s) . . . If she's going to use a release, you want the shorter length so your anchor point is not too far back. Maybe more importantly, the arrows will be shorter which stiffens the "spine" of the arrows and for me at least, made a huge difference in shooting accuracy.
A N D . . . The shorter draw length will create a natural "flex" in her bow arm which will help prevent the string hitting her arm/wrist, and promotes good form.