User blog comment:Herzog de Puce/Baeny, the Mana Engineer/@comment-5781707-20140628064300/@comment-3148314-20140629142952

The AP-ratio on his ult is created for the same reason it exists on other champs: to not waste the AP from e.g. Iceborn Gauntlet. RoA was included because of its high mana pool.

If you want to gain more AP from his ult, I recommend to build Archangels and RoA.