Your mechanic(s) are giving you bad advice.

"As hard as you can" = "I don't know what I'm talking about"

To properly measure the torque you need a custom tool, not hard to make on your own. I've only ever seen one, but it's not really necessary. Just use the basic concepts, get it "tight" and then add 0.25, 0.5 or whatever turns to snug it up. Now, you want to get the cog really tight, but still not as tight as possible.