Thank you for the question!
You're right that the symbol you are asking about is not given a braille symbol in the UEB Guidelines for Technical Material.
Possibly an intuitive transcriber-defined shape symbol (e.g., @$cross) could work for this cross-multiplication symbol. If it appears frequently, assigning one of the transcriber-defined symbols may be advisable. [Our brief webinar "UEB Technical - Underlying Rules and Print Symbols," at time 12:40, touches on transcriber-defined shapes and symbols.]
Regardless of whether you use a transcriber-defined shape (GTM 14.2) or transcriber-defined symbol (GTM 11.2), your description of the print sign is an excellent starting point. (Considering the other uses of this sign, would it be appropriate to include in its description the equals sign which appears to be printed "underneath" the cross-multiply sign we're discussing?) In a perfect world, we would even provide the student with a tactile graphic representation of the crossed arrows (in addition to an excellent description).