Your are mixing up stuff here. The Reynolds number of a flow in the common sense is just a measure of the inertial to viscous forces and is a global non-dimensional number. y+ is a non-dimensional measure of scale based on the viscosity and the wall normal velocity gradient and is thus a local number. This is completely decoupled from the definition of the Reynolds number thus decouple from your length scale. Now the equations @t_bo99 posted are an approximation of the wall normal velocity gradient for the FLAT PLATE. This can be APPROXIMATED by the Reynolds number of the flow. There is no other place the reference length of the Reynolds number goes into y+ except for the flat plate APPROXIMATION.