I would recommend 0.28g at the absolute minimum for that FPS output, but if you really want to cheap out, 0.25g would suffice, if all you're doing is punching holes in paper at a 20 foot range. Actually, at that range, you could just use 0.12g. (not joking.) However, you'll find that as your range increases, groupings become more important, and the tighter your groupings are required, the more you'll have to use heavy ammo.
As for this SS226 being a "quality" airsoft gun, I'm sorry brother, but my experiences with skeleton sidearms products has me laughing at your opinion of their product.