Guild Level

When i check for a "guild war" the system find a guild that has the same level of mine... but how is it determinated? now i'm playing against teams over 60 stars.. and the best in my guild is 40 !!!
I could be wrong, but I think it searches for the guild available to play that's nearest in rating. So yes, sometimes you end up with a really tough series, sometimes a really easy one, hopefully normally a competitive one - there's no fun if they're too hard or too easy.
I don't know if there are upper or lower limits.
According to Pierrot, the system looks for an opposing guild with between 67% and 150% of your guild's total supporters - so star rating isn't taken into account at all.