Actually the base 13 thing is just coincidence, the root cause is the intimely introduction of Telephone Sanitisers into the Gene Pool.
That’s the official version. Presumably I’m not the only person to wonder whether the pan dimensional beings had 13 fingers though?
As for ?, sine is a fairly fundamental function, and sin(?/2) = 1. Hence ? = 2 * arcsin(1). Using the infinite series expansion:
arcsin(x) = x + (1/2)*x^3/3 + (1*3/2*4)*x^5/5 + …
? = 2 * (1 + 1/(2*3) + 1*3/(2*4*5) + …)
There’s probably an easier way, but that’s one way to calculate ? from first principles.