This is great
fascinating discussion.
My personality and soul card are each 8 - either Justice or Strength, depending on the pack - and my Shadow card (by Mary Greer's method) is 17, the Star.
However, there is another way to work out the Shadow of a particular card - lay all the major arcana out in number order in a figure eight, with the upper part of the eight being the cards from the Fool to the Hermit, the lower part being the cards from The Hanged Man to Judgement, and the crossover of the eight being formed by the Wheel of Fortune and the World overlayed. (Everyone got that?!) "Draw" the figure eight with the upper part going clockwise, and the lower part going anti-clockwise.
Now, to find the Shadow of a particular card, track how many cards it is from the centre crossover point - in my case, the shadow of my card (8) is 12, as each is two cards away from the centre crossover. Which in my case is very interesting, as it makes both my Soul and my Shadow cards Strength and/or Justice!
One of my favourite sunlight/shadow correlations using this method is the Hierophant/the Devil <eg>
N.B. the Shadow of The World is the Wheel of Fortune and vice versa, using this system.
peace
hc