Level Setting and REW

I was curious about level setting, well matching actually, and how it is depicted in REW. Let's say after level setting your tweeters by ear, you pull up REW and run separate RTA's for both left and right tweeter. You match the two and they are overlayed on top of each other almost perfectly. Is that confirmation that you have your levels set correctly as the left-right tweeters relate to one another (not as the tweeters relate to your other speakers)? Maybe short of small 1/4 decibel adjustments to fine tune.

Yep, that's about it. REW is a wonderful tool.

Yup.
Also do not worry about 1/4 db. We are not really capable of hearing that. Humans can start to detect changes in loudness around a 3db change.

Having them overlay perfectly after tuning by ear is the best case scenario. Your car would need near perfect acoustics to have the tweeters match in frequency response with no EQ. In most cases, there will be frequency response differences between the left and right side that have to be addressed using parametric EQ. Just use the RTA from the start and avoid the trouble of listening when the system is not yet optimized. Instead of verifying your ear tune with an RTA, you can verify your RTA tune with your ears, if that makes sense.