I'm disappointed with this crossword. How to fill in the blanks will, in general, only be known to those who have studied A-level maths. The title is misleading, suggesting food additives which have nothing to do with it. Also, when you fill in blanks in crosswords such as these, it normally adds something to what is already there, whereas this time the surroundings have nothing to do with it. The cells that have been filled in in a special way are part of the trail of blank cells, which I didn't realize for a while.
Nelson, use the calculator that comes with your computer (in scientific mode). If you have a Mac, look at the bottom row.