According to the Hill's site http://www.hillspet.com/products/pd-...ealth-dry.html
the c/d dry has a dry matter phos of 0.65%
Having not dealt directly with bladder stones I don't have as much insight with them, though I have an article here http://www.veterinarypartner.com/Content.plx?P=A&A=1741
which states that if they are oxalate stones they can not be dissolved by diet & must be surgically removed. The prescription diet is supposedly to re-balance the urine pH to check to see if they are struvite or oxalate.
If she does go in for surgery be sure to have a blood test done before hand to recheck the kidney levels, ensure they will have her on IV fluids during the surgery & there is someone there just to monitor the anaesthesia.