I would say try it without the RTA first. A nice RTA curve can sound badly, and still need refinement by ear. So practice by ear. Start by switching off the auto-EQ and EQ to flat, and spend time finding the best crossover points and slopes, then level your drivers to come close to a smooth sound. You'll probably get pretty close with this and be left with a boomy upper midbass- then try some cuts centered around 250 or 300 Hz.
think in terms of slight adjustments- to levels, eq etc. i have gotten so used to my sound that i can tell when the rear speakers channels have been raised or lowered by 1-db.
i think you can do it it just takes time- also (unfortunately) you need to do it while the Car is running and moving- when it's parked you usually get too much bass because there is no noise cancellation. just be careful and keep your eyes on the road too.