Is it not possible to run the y-web cycle? Then it will automatically locate to the center.
If not, you will have to insert a code like the following after it sets the single surface:
Assuming G54, the macro register for G54 y axis is #5222
#5222 = [#5222+1.0]
Use +1.0 or -1.0 depending on which direction you are shifting.
Hope this helps.