I'm a hopeless chain tensioner. I often get it wrong despite doing it for 30 odd years.
Glory be, I am saved.
The Chain Monkey makes it a doddle. The only thing to challenge me a bit, was working out what chain size and slack I have on each bike to set the tool correctly. The rest is like falling off a log.
Simply pop it on the chain, screw it to the chosen mark and tighten the chain at the rear adjustment.
Chain Monkey Video here
Stock is limited so hop in.