The goal of difficulty adjustment is to submit a share in 10 to 20 seconds. The number of calculations required for share difficulty 1 is 2 to the 32nd power, which is 4,294,967,296.

4,294,967,296 times are calculated in 10 seconds. How many times is 1 second? 4,294,967,296 ÷ 10 = 429,496,729.6 = 0.429G

So 0.429G is difficulty 1.

Difficulty of 1G = 1 ÷ 0.429 = 2.33