Vitamins help tremendously here, and you just need two for each stat. (take a 3rd Protein with you just to be safe)
Take HP, for example. As you should know, Zapdos's maximum HP at Level 50 is 165, but of course, that doesn't tell whether that's with a 30 or 31 IV. Give Zapdos two HP Up and pay attention to how much the HP goes up--if it goes up TWO, then it's definitely a 30 IV; if it goes up THREE, then it's definitely a 31 IV.
This works for every stat except the -Nature stat if it's -Atk (I don't think you're going for -SpD or -Spe.); in that case, the stat will go up ONE for a 30 IV and TWO for a 31. If the IV is lower than that, then you may need a third Protein.
That's the long-winded explanation.
The easy solution is to input the untouched stats on the first line of MetalKid's IV calculator and the stats and EVs after the vitamins on the second line--make sure both lines are Level 50--and that should always give you the exact IVs.
Here's a visual example using a 31/30/30/31/31/31 Timid Zapdos:
-Here's how the stats should be entered:
http://img31.imageshack.us/img31/3154/example1wwg.jpg
-and here's what the result should be:
http://img17.imageshack.us/img17/4512/example2l.jpg
--Indeed, it is 31/30/30/31/31/31.