For those of you who may be wondering, the attachment that had been posted was taken from pg 8 of The Nemeth Braille Code for Mathematics and Science Notation, 1972 Revision (Rule II—§8c). In the same code book on pg 159, there is additional information regarding the use of the multipurpose indicator with a decimal. (Rule XXIII—§177c.) It says that the multipurpose indicator must be used after the decimal point symbol to indicate that the symbol which follows it is not numeric unless that symbol is the comma or the punctuation indicator. It goes on to show several examples. (Further examples can be found on page 203 of the lesson book Introduction to Braille Mathematics.
I think that a careful read through the section of the code book mentioned above, along with a review of the examples provided will answer your other questions.
Even though the answer to be filled into the blank (double dash) is going to be a number, the double dash itself, when it follows a decimal, should not to be considered numeric.
I trust this information is what you were looking for.