How much is enough?

I purchased an 18th century Spanish 2 Reales coin from eBay. When I
look at it (and I've seen a number of genuine examples -- from the
Spanish mint even) it looks genuine. As I put it up on the shelf, it
seemed just a little light. I weighed it (it is 5.53 grams rather
than 6.77) and it is light. It is the correct diameter and all of the
engraving looks correct. It is clearly worn, but all of the legends
are clear and even the main elements in the shield are distinct in
outline, but with wear.

Being 18% light seems like a lot. Should I worry that it might not be
genuine?