What you can do is simply measure two adjacent bolt holes from centre to centre & times the figure by 1.699 (5 bolt holes)
So I've just measured one roughly at around 76.5 x 1.699 = 129.97 - so that would be a std BCD of 130 - I think that's a std Road double!
The BCD is simply twice the length of an arm of the spider (measured from the BB axle to the middle of the bolt hole).
Since you can't readily measure that directly, measuring the distance between adjacent bolt holes and calculating from that is the usual approach.
The triangle formed by two arms of the spider has an included angle of 72° (360° divided by 5) for a 5-arm spider, and obviously 90° for a 4-arm one.
The Sine Law says that the ratio of two sides of a triangle is equal to the ratio of the sines of their opposite angles. The opposite angles for a 5-arm spider are 72° (as above) and 54° (180° - 72°)/2. The two sines are 0.9511 and 0.8090, so the spider arm is 0.8506 (0.8090/0.9511) times the measured distance between adjacent bolt holes.
The BCD is double that, i.e. 1.7012 times the measurement. I won't argue with 1.699, call it 1.7.
