I downloaded the damage calculator and used that.
I put 3 for the Sp.A stat, 1 for the offensive nature boost, used 2 for Rhydon's Sp.D, and used the 2.25 modifier.
If I put 4 for the Sp.A stat and 0 for the offensive nature boost then I get the 24.75 that you got, I'm not sure which to use.