Just for reference, I did a quick Excel chart (better than SE's!):
http://i59.photobucket.com/albums/g2...Zero/zrate.gif
So.. if you parse at 50% accuracy on a given mob, assuming you had 10% Zanshin proc rate, you'd gain a whopping 2.5% to your parsed accuracy. So you'd be parsing at 52.5% accuracy.
50% is, by far, the most "efficient" parsed accuracy rate to make the most of Zanshin, but since that isn't very practical, let's look at something a little more typical.
If you parse at 75% on a given mob without Zanshin, you'll parse at 76.85% with 10% Zanshin proc rate. If you parse at 85% on a given mob without Zanshin, you'll parse at 86.25% with 10% Zanshin proc rate.
And this is all assuming a very generous 10% Zanshin rate. You can see what happens with more realistic proc rates.