I'm trying to fix up an old bike that had knackered shifters and broken derailleurs into an emergency one speed. I'm waiting on a cassette kit and proper cog from ebay, but in the meantime I've shortened the chain and lined it up with the middle chainring and the corresponding rear cog. Seems to work fine (but I wouldn't trust the old cassette cog at speed or uphill). The frame has horizontal drops, so I've got about an inch and a half of leeway as regards chain tension. How do I know when the tension is right? I've posted a very bad picture if it's any help.