The symbol := means "is defined as equal to" but unsure of the correct spacing for the symbols in braille. I have included the surrounding text and several samples of spacing. The first sample is the entire equation (please let me know if there is a discrepancy in the division) followed by samples of the area in question.
Even wondered about the dot 5 between to show they are side by side.
Because the symbol has been defined to mean "is defined as equal to" it is one sign so should not be broken up.
If the expression fit on one line, there would be no space before the colon. A punctuation indicator would precede it. So it will be a 4-cell symbol, with a space following the equals sign. (456, 25, 46, 13, space)
This expression does not fit on one line. Our division rules say to divided before a comparison sign on the baseline of writing. I feel that the first runover line can begin with the punctuation indicator. This is an unusual formation so you should identify the symbol in a transcriber's note. Something like this: "The symbol ___ means "is defined as equal to". It is printed as a colon immediately followed by an equals sign."
I see no discrepancy in the division sites you have chosen. I am tempted, however, to make an exception to "displayed" format here and start the expression in cell 1, after a blank line. This will allow you to keep the "C" portions together on one line. This is breaking a format rule, but there are times when it benefits the reader to do so.
I think you need to insert a space after the word "Gain" as I believe it is a function name.
See the attached brf file for my suggested transcription of this long expression.
I invite anyone to chime in if my solution is problematic.