Chain wear tools - recommendations?

I recall that the last chain I had went beyond its recommended stretch levels before I changed it and I'd like to avoid this happening again. I should get a chain wear tool so I don't end up using a knackered chain for too long.
Having looked at the tools available to check for chain wear, it seems there are some basic ones for around £5-6 and then some more sophisticated ones going for around £20 then a big step up to digital ones for about £60.
Are the cheap ones effective or are they a false economy? I'm not desperate to spend a lot on this but neither do I want to end up with something that is effectively useless.
Any thoughts from someone who uses such tools would be much appreciated
Having looked at the tools available to check for chain wear, it seems there are some basic ones for around £5-6 and then some more sophisticated ones going for around £20 then a big step up to digital ones for about £60.
Are the cheap ones effective or are they a false economy? I'm not desperate to spend a lot on this but neither do I want to end up with something that is effectively useless.
Any thoughts from someone who uses such tools would be much appreciated
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http://www.amazon.com/dp/B000OZFIMQ/ref ... B000OZFIMQ
Parktools :?:SheldonBrown
Would I ever need to know if the stretch is 0.30% rather than, say, 0.40%???
http://sheldonbrown.com/chains.html
...scroll down to "Measuring Chain Wear"
chains are inch based, so a 12" ruler is simplest, but the instructions also cover using mm
http://app.strava.com/athletes/1920048