I used to think i was a bit of an expert on this subject until Smidgen disappeared earlier this week. Now I know that it really depends on the cat. If he is older, and is chipped, then I think you are doing the right thing with the supervised runs. As he managed to get lost his first time out, it might be useful to make sure he is trained to come to a call or rattled box of biscuits. If that is the case you can let him go over the fence and gradually expand his territory, always knowing how to get back. It really is a case of trial and error... they are such independent cusses when they want to be
The first time he escaped was the evening back from the rescue centre, did not have time to settle in at all. It was my fault as my previous cat was disabled some what with only 3 legs an no tail. So he used to spend most of his time in the house or garden as he could not get up on the wall or fence around the property. He never jumped up on window sills either.
So of course when my new cat "BLUE" arrived I really did not think, I shut all windows except our toilet one. I was just ajar but BLUE managed to find it and jump on the toilet seat,then the window sill to escape. Just below the window was our porch roof so easy way down. He was missing for three weeks, until someone reported a stray 3 roads away, thank goodness it was him.
So you can see how I worry about letting him out, but I must pluck up the courage to do it sometime