Calculation in editable form

Since your calculation involves division, the Simplified Field Notation (SFN) is not recommended. A custom calculation script is a better solution because "FIELD C" might be empty or 0, which would cause a division error. Here's a recommended script:

var a = Number(this.getField("FIELD A").valueAsString);
var b = Number(this.getField("FIELD B").valueAsString);
var c = Number(this.getField("FIELD C").valueAsString);

if(c != 0){
 event.value = (a-b)/c;}
else
 event.value = 0;

Ensure you are using the right formula:

a - b / c divides b by c first, then subtracts the result from a.
If you intended for (a - b) to be divided by c, enclose it in parentheses: (a - b) / c.

Format the field as a percentage with 2 decimal places:

Right-click the calculation field and select Properties.
Go to the Format tab.
Choose Percentage and set Decimal Places to 2.

You can't have spaces in field names in a simplified field notation without escaping the spaces.  Try this:

Field\ A-Field\ B/Field\ C

Or you can do a custom calculation script instead like this:

event.value=this.getField("Field A").value - this.getField("Field B").value/this.getField("Field C").value;

Format the field to percentage with 2 decimal places.