Let me see if I understood your question.

   You want to know how to say to yourself or somebody else how to set the decimal

places show in your display (in a calculator/in Excel etc)?

   Or you want a sentence to ask somebody (co-workers, classmates etc) that you want your calculator set to some "fixed" decimal places?     Something like "Please, let´s set  (or fix) our numbers to two/three/four...ten decimal places." or "let´s set the display to show numbers/ to round the numbers to 8 digits (e.g. using the "fix" key or another given key).

   Note 1 )

 "The  FIX key permits the rounding of any decimal from 0 to 8 digits (6 on an 8-digit calculator).  Rounding will continue until the NORM key is used.

              The FIX mode affects the display only. The numbers for internal calculations are not rounded off maintaining accuracy."      (Source: Computing With the Science Calculator - Ed. Casio)

  in use:

               FIX  2           3.14

               FIX  3           3.142

               FIX  4           3.1416

               FIX  5           3.14159

NOTE 2)  

    Maybe (certainly) there are other ways  to say this, so let´s wait more contributors, this one was a kick-off of sorts.  Anyway, I hope it helps.