Other harmonics spots may work well to , but it's usualy trial and error , till you find something that sound nice ... It's a matter of experience , when you get used to it , you'll do it without even thinking.
As for tapping , usualy it starts within the scale on one string, and sometimes EVH just copies the same pattern to other strings , which is out-of-scale of course.
Anyway , you should have a few tap lines in your pocket to use whenever you want , like min/maj arpeggios that fits over their relative chords , or pentatonic lines.
