I have tried using the intersection of the longest distances, but I keep getting different results.
This is normal, and not a problem. What tends to happen when you are trying to find the approximate centre of an uneven shape is that crossing lines taken from various points on the circumference, or edges, yield a small clear area.
Once you get to this stage you can just eyeball the 'centre' quite accurately.
Even if your circular piece of wood were much more uneven than it is you can use this technique to get an acceptable result since it's so easy to guestimate the centre of this much smaller shape left by the marking out.
If you'd prefer a different methodology so you can 'weight' the circle within the uneven shape visually, simply draw a 14" circle1 on a piece of paper or card stock, cut the circle out with scissors, place on your redwood and move it around until you like how it looks. Make a mark through the central hole and you're done.
1 If you have no compass large enough, just make yourself a rudimentary beam compass from a strip of thin wood (even stiff card will do). Drill/poke two holes in your beam 14" apart, pin it to the paper through one hole using anything suitable, push a pen tip or pencil through the second hole and rotate. You can use this or a "string compass" to draw circles of any diameter you'll ever require.