According to the Manual of Close-up Photography, at that magnification, you need to increase exposure by 4 stops. Be careful when stopping down, too much and you will get diffraction. Enlarger lenses do work very well as macro lenses. Some longer focal length lenses will not function properly for cloe-up or macro due to their construction. Reversing a lens is most times a better way to achieve the results you are after.
"Manual of Close-up Photography"
i will get a used copy of that.