This may work on printers other than a Robo3D/R1/R1 Plus
The 8mm rod(s) are 24 mm shorter than your glass length.

** Warning remember your working with glass! be sensible in the amount of correction you try to make. If you apply TOO much shim you could break the glass. I some cases the glass is between $30-$50.00

When I created my 330mm Y Rails upgrade for the Robo 3D
http://www.thingiverse.com/thing:1154745
I learned that my bed glass was .02 mm lower at a certain point that at other points.

When your auto leveling a larger printable space it's expected that you will find larger variations that when using a smaller area. I personally have my auto level routine in the firmware reaching out to points at 5 mm and 305 mm when it auto levels the Y area (this is a firmware config change).

Ok now for the fix.
If you did the 330mm Y rail upgrade, you will have the old standard length rails. I created the (printed in red in the pictures) rod holders to use these standard length left over rods for this design.

Print 2 of the blocks and place one of the left over 8mm rods in the holes and slide the slots onto the glass. This gives you a support bar which you can lift your glass in a location that has a low spot. I used some washers I had laying around to accomplish this shim action.

If you don't get the right lift to get you a perfectly level bead area, print 2 more blocks and use the 2nd 8mm bar you have.

I personally printed mine is PLA because the heated area does not extend into the area where the block sits. IF you upgraded to a 300 mm length bed heater, you had better use ABS or nylon which has a higher glass temperature.