Get the "Gold Ring" over the X & Y Coordinates of the Tripod centreline. To help, use the centre focus point through the view finder with the camera tilted 90 degrees looking down onto the tripod centreline.
You are trying to set the lens at the No Parallax Point (NPP) not the Nodal Point. The Sigma 8mm is very close on the gold ring although some tiny adjustment will be needed for perfection. The Gold Ring is close enough to get you going with minimal errors.
Another process is to use a "Plumb Bob" weighted with fishing line around the thin groove at the front of the kens.
Be 100% sure you slide the "complete" lens cap of the front of the lens (it is made of two pieces)
Set the aperture at F/8.0 and adjust the shutter speed to match the internal light meter -2EV|---|---|+2EV the centre is 0EV and for this the camera should be set in full (M) manual mode. Make sure the ISO is at it's lowest numeral setting (especially on the Nikon) ISO 100 would be correct for a Canon DSLR.
Shoot RAW (.nef) so at least you can rescue if needed. If your camera can do .nef & .jpg at the same time all the better.
Good luck, I have offered my best advice.