Need help leveling a Rega P25

I just installed a Rega wall shelf for my Rega P25, and though I tried to be careful, it is not 100% level, though it is pretty close. The right side and back need to be raised slightly. I've never dealt with audiophile tweaks before, and I'm wondering if there is something better than, for example, simply putting little pieces of cardboard underneath the rubber feet that need to be raised. Any suggestions?
I recently leveled my Planar 2 by using a thin adhesive felt sheet, cutting a long strip a little wider than the rubber foot and folding it over on itself the proper number of times to get the right thickness. The adhesive keeps it stuck together when you are done, unlike a stack of business cards, etc.

The felt seems like a good non-resonant material, that's what my turntable mat is made out of.
try loosening the right side ( and back ) screws a touch..Just enough to lift up the appropriate side ..You may need a second pair of hands to hold it up,so you can tighten it once you get it up to where you want...Sometimes there is enough play in the bracket holes where you can move the low side up maybe 1/16" or so..You could also possibly try a longer and slimmer screw on the right side to allow to give you more "doodle" ( between the holes and bracket ) room to move it a touch...Hopefully that can give you enough to get it to where you need it...Hope that hepls.
Wooden shims from home depot will do the trick with the shelf. Don't start putting junk under the feet of the table or you are asking for a table that's never right.