Do you mean 0.08" rather than 0.008"? 5/64" is a typical value for setting the action of the low E string, measured at the 12th fret (not pressing any frets). I often go up to about 6/64". 0.008" is more of a value for setting relief, but not 5/64, that would be way too much!
0.010" is the typical value for setting the relief, measured at the 7th fret while pressing the first and last fret. A few thousands of an inch either way is not critical and will not cause the guitar to buzz severely at only one fret. You can go over 0.010" if it gets rid of the buzz and you can live with the overall action/feel. You can't really loosen the truss rod too much unless you really start cranking on it. Mgruff's advice to take a little at a time is good.
I haven't tried a credit card for a fret rocker, but if the card is perfectly straight it should work. You will need to get the neck as flat as possible to find the offending fret(s). Set the card across only 3 frets at a time until it rocks, then you know either the middle fret is high or one of the outer frets is low. Make sure the rocker is perfectly parallel with the string path so the fretboard radius doesn't cause a false reading. Be cautious using "straight edges" as most are really not that straight (hold two of them together to check). We are talking very very small differences in fret height so the straight edge has to be perfect. I have a fret rocker from StewMac that is supposed to be machined perfectly straight.
It does suck having to send a guitar back. I have had to do it a few times, at least once for a really low fret. I would rather have a high fret or two because it's much easier to fix.