Angrybob strikes again.
Otto Stone is not nerfed but the math behind the calculation is a bit cryptic for the uninitiated.
To sum this thread up I used the stone at lvl 14 on a 3rd life character which took me only to lvl 17 with maximum xp buffs going (50% pot, 10% VIP, 5% voice, 5% shipbuff). Since from earlier experience the stone can take you into lvl 19 I couldnt figure out why.
Explanation for my error;
The stone gives you enough xp (unbuffed) to reach lvl 16. Which means it only gives the difference between your current xp when using the stone and the xp needed for lvl 16. Xp bonuses are calculated on the added xp. So the longer you wait to use the stone past lvl 8, the less xp you get when bonus xp is added.
Conclusion. Stone should be used when lvl 8 is reached and every xp beyond lvl 8 will give you less total xp.
Final conclusion, Im not a rocket scientist :-)