If you err do it on the short side. In this case I would say go for a bow that is 27", but adjustable with the module. Most bows will draw longer than they are marked so be aware of it and have the bow measured also. Don't take the sticker's word for it. Depending on the quality of the rigging the string and/or cable will creep over time resulting in a slightly longer draw also. How much depends on a combination of string material and quality of workmanship.

As for the wingspan method of determining your length, ask yourself. Do I have longer fingers than most? This can result in a slightly longer bow than you should have. The wingspan method is not perfect, but neither is any other way.
To use a light bow and measured arrow the person drawing it must know something about shooting form and be able to anchor consistently and at the right place. Most people stretch out and anchor too far back and shop personnel don't tell them this. Then there is the idea of drawing the bow with fingers which will not compute ptoperly if he will ultimately be shooting a release. In this case it's normal to reduce the measurement by anywhere from 1/2" to 1".
See, there's a lot to it. That's why I like bows that are adjustable for the shooter. You may get a bow set at a certain draw, shoot it for a while, and decide that it needs adjusted some one way or the other. If you have one that is not adjustable then you're stuck with what you got.
Such bows have their good points, but IMO, should be left for the experienced archers who really know what they are doing.