30/30 Reticle

[scwhitetail]

So I've heard discussion lately about estimating range using a 30/30 reticle. Can anyone explain how that works? Its my understanding that the reticle is 30" at 100 yards and you can do some math to estimate the range, but I'm not sure on the specifics. Also wouldn't zoom affect the size of the reticle and therefore the range?

 

[Tempest 455]

The thin bars in a duplex reticle may also be designed to be used as a measure. Called a 30/30 reticle, the thin bars on such a reticle span 30 inches at 100 yards when the scope's power is at 4x. This enables an experienced shooter to deduce (as opposed to guess or estimate) the range within an acceptable error limit.

http://en.wikipedia.org/wiki/Reticle