Setting chain length My chain broke and in a moment of silliness (I was cold wet and slightly annoyed)  I threw the broken one away before thinking about measuring the new one.

Is there a guide to getting the chain length/tension correct ?

It's an 8 speed road bike (Shimano Sora back, double ring front)
Put the chain around the big-big gears without threading it through the derailleur. Have the chain overlap by one link. Now with that length thread it through the derailleur.

http://sheldonbrown.com/derailer-adjustment.html#chain