How to divide signal level from function generator
Let's call the top resistor of a voltage divider R1 and the bottom R2.  The output impedance is R1 // R2, and the gain is R2/(R1+R2).  Set these two equations equal to what you want.  Now you have two equations and two unknowns.  Solve accordingly.

Note that R1 encompasses the 50 &Omega; of the source, so the actual top resistor (what you add directly in series with the signal generator) will be 50 &Omega; less than what you solve for R1.  At your very low gain, this will make little difference though.