Around 50 is okay, but the closer to 100 it is, the more accurate it is, with 50 being a good point to start.
And, the best way I can think of to get an estimate of your Zapdos's EV's would be to give it to Emerald if you can, and write down how many of the berries that take away ten EV's it takes to make it so it has no effect anymore for each of its stats. If you don't have Emerald, then you could try inputting average IV's of 15 for each of its stats and see what comes up, but these numbers could be way off if your Zapdos has any really high or really low IV's.