Initially you should be late apexing every thing. that will give you the slowest corner entry and the fastest corner exit. Once you get that down, then you can start exploring when an early apex in a string of truns may be usefull.
BTW, On a race track, the dimentional apex of a turn is never going to be the fastest way around the track. your either going to late apex or early apex, and 9 times out of 10 your going to late apex.
Good article on the subject
The 1st drawing shows the "dimentional" apex, and then 2nd one shows the faster late apex.
Regardless, cornering theory dictates that the best line is the one that gives you the longest acceleration zone coming out of a given turn.