I always love formulae where the most important variable is a loosely defined "fudge factor". I'm surprised that one important variable is missing from the formula - mass of the camera / lens combination (higher mass means you can shoot at slower shutter speeds than a lower shutter speeds due to the natural damping from inertia).
Of course, then there is my favourite solution - get a good solid tripod and suddenly exposures that run into many minutes are quite doable.
I'm not 100% sure about long time firearms shooters making for good long shutter speed photographers, if I look at my brother-in-law, a long time gun club member who does target shooting with rifle and pistol. Those skills don't always seem to carry over to slow shutter speeds shooting with a camera.