Using the smallest aperture on your enlarging lens (and a translucent white filter if you have one), expose a strip of enlarging paper as you would a test strip for a print. (do not project an image for flashing). Use 1 second intervals. When you've developed the strip, look for the first change from white to palest gray. Any of the times before that one can be used to flash the paper without it registering a tone by itself.