Thanks for your feedback. I was wondering about that as well when I was going through the numbers but I think this is right. Let me know if I have missed something. Here is what I did:
If there are 67,184 total Beta cards, I divide them by 750,000 for the total plots. Now that is 8.96% like you write. However, this means only that there could be 1 Beta on 8.96% different plots. If we put 5 Betas on each plot, we have only one fifth of that, so 13,437, which is 1.79% of the total 750,000 plots.