Alan, I leave the negative in the enlarger and put two clear plastic cups over the lens. This diffuses the light nicely. As Thomas said, find the time just before that which gives you tone. If using FB paper you will need to dry the test strip first before you make your judgement. Pre-flashing is for negatives with stubborn dense highlights or highlights which are too hard to burn in. Results with pre-flashing are normally much better than burning in however. A great example of a neg that would benefit from pre-flashing would be an well exposed interior near a window. Pre-flashing would bring detail back into the window panes, items sitting on the window sill, etc.