OK, let's see if we can figure out the pattern here.

0-50 - Wizard's Slave
51-100 - Silly Adventurer
101-250 - Devoted Knight
251-500 - Royal Heir
501-750 - Great Oracle
751-1500 - Powerful Wizard
1501-(?) - Magical Genie
So we have approximately:
50
50
150
250
250
750
Which gives us something like:
(N/A)
+100%
+150%
+100%
+50%
+100%
So, er.
Peace & Luv, Liz