chain tension

My neighbor is lending me her 6 ft. giraffe to try out. She hasn’t ridden it in
many years…it’s pretty old and beat up, so I’m giving it a good once over
before I try it out. One thing I noticed was that the chain tension seemed a
little sloppy. So loosened the axle nuts and adjusted the two screws that
position the axle until the chain was tight. Now I’m worried that it is too
tight. There is no droop to the chain when the uni is laying on its side. There
is a bit of vibration when I spin the pedals. I don’t want to break the chain or
wear out the cogs…

Any rules of thumb for judging chain tension?

-Rick Bissell