With a sturdy setup (tripod, copy stand, remote release etc.) longer exposure times are no problem.
As for exposure, set aperture to f/8, ISO to its lowest value and expose up to one or two stops longer than your exposure meter suggests. You can also bracket a series of shots and see what NLP delivers based on each exposure.
I found that NLP is fairly tolerant to exposure (see below), nevertheless, it’s a good idea to keep the histogram close to the right edge, but not too close. Here’s an example of a series of shots, taken while increasing exposure by 1/3 stop from shot to shot. The highlighted negative was shot at +2/3 stops, the two rows of converted images were made by NLP with different conversion settings.
