It seems that when I am in the big ring, the front der. downtube will slip so that eventually, the chain will rub on the deralleur cage. The solution is tightening the wingnut on the shifter. However, I am concerned that I am tightening it too hard. How hard is "too hard"?

weeeeeee!




(ps: bikes don't have wingnuts )

Make sure someone didnt lube the shifters, that has happened to me.
Otherwise, too tight is when you cant shift any longer

Or something breaks or strips.

Bikes that have friction shifters often do.

Your shifter probably has a number of unique-looking washers in it. Those have to be installed in the right order. If the rear shifter is working OK, you might take it apart to use as a template to see if your front shifter, which slips, might have been assembled incorrectly.