If the standard length of a B.G. crossing is 597 cm, the number of crossing, is

ANS:B - 1 in 12

Here is the full explanation for this question.

It's not based on any formula but rather directly on RDSO and IRCEN values given in their publication known as "Turnout" wherein if you see Annexure 8 and Annexure 9 table and see the values of Length for Built-up Crossings(not CMS Crossing) you'll see that they are 4800mm and 5970mm for BG for 1 in 8.5 and 1in 12 respectively.

Also, for MG it's 4800mm for both 1 in 8.5 and 1 in 12 (for Built-up Crossing).
So for BG @4800mm - 1 in 8.5,
For MG @4800mm - 1 in 8.5,
For BG @5970mm - 1 in 12.
For MG @4800mm - 1 in 12 here.