by correlation he means partial phase cancellation.....
do you never use a correlation-meter?
If you're 4-tracking there shouldn't be any serius problems (with 1 mic!), just check your mix in mono.
you get a wider stereo-field by using a different amp on the right side then on the left side....that'll make your stereoimage seem way wider, but you're not getting the same effect as with the other technique (amp A 100% L/R, amp B 80% L/R).
trust your ears more than your meter, mix in mono and listen for too much cancelation....the only time I use a correlation-meter is in mastering when I'm using stereo-enhancers....and that's VERY rare.
and: a short dip into the negative area doesn't mean anything , just that there are parts cancelled but that happens all the time with waves interfering (even if they're from a completely different source) just watch that it's only rarely going into the negativ area and not too deep.
use your ears to check mono-compatibility