You can also pre-flash in camera with a bit of white plex/whatever in front of the lens.
Meter through the 'whatever' and give a white-light exposure xx stops less than metered. This works with pinhole or lensed cameras.
Flashing for paper negatives is very different than flashing for paper prints. For a print you want to flash to a standard "imperceptible almost-but-not-quite-pure-white." If this is what you want to do because of hidebound tradition then give 2 1/2 stops less than metered.
However, when pre-flashing paper negatives it is better to flash to a density of 0.2 OD to get you well off the toe and on to the beginning of the straight(ish) bit of the HD curve. For an 0.2 OD flash give an exposure of 1 2/3 stops less. [Obviously not what you would normally want when making paper prints, but not a bad idea if you are making enlarged display transparencies from negatives where shadow detail is no problem but holding highlight detail can be the devil.]
The above exposure figures are for Ilford MGIV RC. But they hold well enough for other Ilford VC papers. They also assume you have calibrated your system so that a metered exposure of a white wall will result in an 18%/ZV tone.
You can also post-flash - the order of the two exposures is rather immaterial.