I use a little desk lamp with a dimmer wired in plugged into an enlarger timer and hundg on the wall for some distance. It's a very simple procedure to lay a sheet on the table under the lamp and give it a preflash bump before loading it into the easel.

For what its worth this is the very same technique used in graphic arts predigital while making halftones. They required a preflash to bring up the shadow detail and correct for ink bleed in the printing process. The time was standardised for all shots as it was just to bump the dot formation over the threshold exposure.