You need to pick points based on the spells speed and damage units there.
So for example, if your spell's speed is 1000 and it covers 1000 range, you should pick every 0.2 seconds units around a point which is calculated by Distance/Speed*Time, so if its the first point, it will be 1000/1000*0.2 = 1/5 = 200, which means you will pick every units at approx 199 range from a point 200 distance from the caster towards the targeted point.
Of course that as the time between picks is smaller, the accuracy gets bigger.