Well, you can try this to be more precise with your screams...
Play a little run, like one octave of the pentatonic scale
|---------------
|---------------
|---------------
|---------5-7---
|-----5-7-------
|-5-8-----------
On the approach of the last note, slide your index finger off the pick, replacing it with your middle finger. Now, image that the frets on your guitar extend past the fretboard and all the way to the bridge. Find the fret which would be 24 frets up from the note you want to scream (which in the case of the 7th fret would be the virtual 41st fret, which is about halfway between the end of the fretboard and the bridge on my guitar). Place your index finger lightly at that spot and pick the note. It should scream quite nicely. I hate using my thumb or the flesh of my palm to execute harmonics, so I use this method. Works every time with some practice.
Start easy... fret the 1st fret and the virtual 25th fret, which shouldn't be hard to find since it's not far past the end of the fretboard. Move up note by note and you'll eventually start to just know where the spot you need to place your finger is. Hell, I can scream every note of a Dm9 arpeggio with this method.
"You must stab him in the heart with the Bone Saber of Zumacalis... well, you could stab him in the head or the lungs, too... and the saber, it probably doesn't have to be bone, just anything sharp lying around the house... you could poke him with a pillow and kill him."
- Aqua Teen Hunger Force, The Universal Re-Monster