So plussing adds damage as well? I thought it just added to the "hidden" stats on the item (in this case, Magical Attack & Crit Rate)
I'll go take a look for character plan tho.. If it works, it should have the math in the source.
Both damage and some (not all) ghost stats. And, yes, the math is pretty easy to see there. Here are some numbers for your Raksha.
Non-OD
+0 = 2194
+6 = 2479
+12 = 2918
+16 = 3466
+12 T9 = 4234.4
+16 T9 = 4782.4
OD
+0 = 2632.8
+12 = 3501.6
+16 = 4159.2
+12 T9 = 5081.3
+16 T9 = 5738.9
The bonus per plus level is not linear, by the way. It is much more pronounced on higher category. First 6 plussing levels gave you only 285, then the next 6 give you 439, then the next 4 give you 548 more.