If you want the best quality then sto down the lens for 2 stops. So a 5.6 lens it's working range is F11 for best quality. (leave it there..)
Second you must optimize you negative for best print quality (range of gray levels). Someone mentioned 15 secs. This is possible, but could also be the cause of a denser negative. What works for him, is not sure it will work for you...
At last a print time comes out of all your variables. Maybe i would use lightbulb replacement with a lower value or take a longer lens so the distance is longer between lens and paper (increase time). A second benefit is that when using a longer lens, you use lens part of the lens glass (only the middle part) so the sharpness will increase a little bit.
I assume you want longer times to work on your image (burn)