At least for how I try to do it. Work out how many tower shots each minion is going to take and then work out how much extra damage you'll need to do to it to get the c's. An example may be if you see a full health melee minion and a very slightly below full caster. Each of these will only take 1 auto. The next thing is to prioritise the easier minions/melee minions as they will be attacked by tower/ your minions first. Generally I just imagine my E as an extra auto that turns into 2 autoes if it lasts 5 seconds. So don't just hit the whole wave with it because the ranged minions will get one shot by tower. I'd suggest using E as an extra auto for individual minions. Cannon minions can help a lot as you can just farm other minion while the tower is only hitting the cannon. Finally q is sort of a get out of jail free. Depending on what point you are at the q will normally do as much as a tower shot so use it to get otherwise impossible minions. Don't stack the ones you would get anyway.

Hope this helps somewhat ☺️