I've wanted to get into paper flashing so now is as good a time as any to learn a new trick!
I hardly ever print, but when I flash a print I've generally done it with a small flashlight (continuous, not a flash) bounced off the ceiling. In my scheme, typically about 1/2 to 2 seconds does it. You need a switch that goes on/off quickly, when you want it to. What I do is point the light to a part of the ceiling that can "see" the full sheet; this illuminates it evenly enough for my purposes. I find the approximate best time with several exposures on a test strip. First find a good printing exposure (where the whites are a bit too light). Then I'll cover 2/3 of the test area with a card and give about a one-second flash. Then cover only 1/3 of the area with a card and give another one-second flash. So what I have is a section, each, of: straight print, one-second flash, and two-second flash.
You can fine-tune another flashing test from there. I just count off the time, which is mostly good enough for me; I don't see the flash exposure as being that critical. But if I was doing more critical work I'd probably set up a better controlled system. It's useful to have a white border as a reference so you'll know if you overdo the flashing.