I get Q at levels 1,3, and 5 and poke with it as often as possible and then after 6, max her heal as soon as possible. She can sustain herself and her ADC well with rank one heal if you guys aren't eating every initiate or heavy poke. This let's her poke, which it seems around my skill level people don't expect, which make it fairly easy for me to push them out. I only heal my ADC if he has less HP than me and I'm not below 40%ish (unless I'm about to back).
I gauge how well I've done by how much my ADC has a CS lead by the end of lane, and how many times I've been harrassed in all chat for cockblocking the enemy team. After the game, I aim to out heal one of the enemy damage dealers and if I didn't accomplish that than I didn't do as well I would have wanted (on top of warding the most, assist count, how much I died, basic stuff).
I will let an ally die without a second though if they are taking way too much harass or initiates when I feel like they normally would have if I wasn't Soraka. I'll fix mistakes, but if it seems like they are going hard because LOLHEALERCAN'TDIE then I'll heal when I have full HP and spend more of my time and mana harassing. I don't like putting myself, especially with a 10% max HP cost, in a position that will get me killed at the cost of people who should and could be avoiding things. If I die, I don't get XP, I don't get gold, and I can't help people with a global ult when it could change the game.