The spell description is "They flit around you to a distance of 15 feet for the duration". So that's 15 feet from your space, not 2.5 feet inside your space and then another 12.5 feet. "You" are not a point-mass, you are a 5x5 square (assuming medium or small). It seems it's just like any other aura. Is the foe within 15 feet of me? Yes? Then they are in the effect. No? They are not within the effect.
if you take from the "point" inside of you, the sphere will have a diametre of 30 feet. If you take from the square as you suggest, the sphere would have a diameter of 35 feet... so are you sure about that? I can definitely see the logic in your proposal...