Ernest, you are right.
They base 1918 on the idea that it is 3.5 years after Jesus was made King in 1914 due to some speculation regarding Jesus himself.
Then they conclude it must have occurred prior to 1935 because Rutherford came up with the idea of the identity of the Great crowd as being the other sheep. They claim this new revelation was likely communicated to them by the resurrected anointed.
However, with all this speculation they overlooked one thing. They make the bold statement that the 1935 great crowd revelation was definitely communicated to them. Since it wasn’t made up by Rutherford and it wasn’t from the bible, they admit to it being revealed to them somehow, they just aren’t sure who communicated it to them.
So we get back to the OP’s question. How does the GB get enlightenment? For some things they do know, for others, they just don’t know and resort to speculation.