Wobbly cranks/tightening with a household tool?

But what is considered over tightening...?

If you are using a piece of scaffolding for extra leverage that's probably too tight.

What kind of torque is recommended...?

About 40Nm I think, although I don't have a torque wrench that goes this far so am not speaking from experience. The crank manufacturer will most likely recommend an appropriate torque.

I go for as tight as a possible by hand then a firm body weight final push down. Is that too much?

That's about the same as I do and haven't had any problems yet.