Attempting to Calculate Percentages but "The value entered does not match the format of field."

Hi, everyone! I have created a fillable PDF document using Adobe Acrobat for assessment purposes within an academic department. I am needing to calculate the percentage of students who attained a specific learning outcome.  I have the # of students assessed and the # of students who attained the SLO.  While I have spent all day attempting to find a JavaScript formula to calculate the percentages, I finally had a breakthrough...or so I thought.  Eventually, I can see the percentages populate but only after I click "The value entered does not match the format of  the field [PercentAttainedAssessed Row1]" OK box umpteen times. I have no earthly idea how to do JavaScript; thankfully, I've been scouring these and other similar boards to help me out.  Would someone be willing to assist me (a novice) in figuring out what I need to change to make this box disappear?  I have ten different versions of these forms to complete!  Any and all help would be greatly appreciated.