If you are like me you probably have searched for clues to and found little... particularly when confronted by the errors as you unravel how this works.  Okay a slight exaggeration, there are snipets of info that people have solved this annoying issue in  making comments work in Drupal 8 REST services. The lack of collective information in resolving issues has meant hours of debugging and working through core code to understand what is actually happening.  If you want to...

Adding a floating decimal point for n number of places is quite easy to achieve.  The key to understand is the receive an outcome of a fraction the numbers need to be Double.  So Int numbers will result in a whole number product.  By way of example: 80 / 3 = 26 We know that this result is incorrect.  To get the correct outcome the equation needs to be as follows: 80.0 / 3.0 = 26.666666666666667 If you have a whole number - add Double to the whole number.  So...