Your question about pollination of avocados is rightfully in the "Science" category, but it boils down to essentially a math problem. Some of the aspects do depend on the biology of the avocado tree, in that avocado tree flowers have only one stigma (one of the female parts of the flower). Given in your question are some of the other variables--150 flowers (and therefore 150 stigmata), 80% pollination rate, and 8 grains/stigma found on average. What is not clear is if each stigma received the pollen, whether or not the pollination was successful.

For the case where only the successfully pollinated 80% received the pollen, and using the above figures of 150 flowers, 80% pollination and 8 grains/flower, the number would be 150 (.8)(8) = 960 grains total.

If all stigmata receive pollen, the number becomes 150 (8) = 1200 grains total.