Snowballing a champion

I played a game last night with Ahri (one of my favorite mid lane champions). In that game, I got an early level three kill on the Twisted Fate in lane with me. I got him down to about 1/3rd health and then jumped in with Flash, Ignite and Orb of Deception which finished him off (I probably didn't actually need the Ignite but wanted to be sure).

And then I did something stupid. I finished off the minion wave and got some hits on the tower. The problem is that early hits on the tower are useless at that low a level. I was auto-attacking for 56 damage about once every 1.3 seconds against the 200 armor turret. So my five or six autoattacks did about 100 damage out of 2200. That 10 seconds or so could have been much better spent. The decision tree needs to be based on your champion and your mana situation.

1) Is your mana sufficient to gank?
   A) Yes: Call for your jungler to double gank top or bot with you. Try to either get another kill or get a kill on your ADC, top or jungler and an assist.
   B) No: Go back to base. Does your champion have good AP scaling on abilities?
        I) Yes: Buy an AP item (hopefully Blasting Wand but maybe Boots + Amplifying Tome). Now, look at the map. Is an enemy lane pushed?
             a) Yes: Go gank it
             b) No: Go back to lane and try to pressure the guy you already killed. But a ward because you're going to push.
        II) No: Buy sustain items (mana regen, spell vamp, boots, etc). Now look at the map. Is an enemy lane pushed?
             a) Yes: Go gank it
             b) No: Go back to lane and harass with abilities and take advantage of your significant edge in sustain

As you can see, many of the scenarios involve ganking which I failed to do. As it turns out, I had about 1/3rd mana and a faerie charm already in inventory. I should have immediately called for a double gank top and tried to snowball. Instead, I made the useless hits on the tower until TF came back (probably knocking it down 300 health or so) then cleared the next wave and went back and bought. I got back to find their Soraka now in lane with me 2v1 and me unable to do much (essentially nullifying my level/gold edge on TF). Our bottom lane probably should have done better with their own 2v1 but the fact remains, I screwed up in not roaming as soon as I got the TF kill.