If you want more than 2 decimal places on a field (which is what I think you are asking) then you need to clear the AutoFormatType property (if set) and set the DecimalPlaces property to 5:5 - to format to 5 decimal places for example.
Note you can only format up to 5 decimal places in a field in NAV, you can ream more about this property here...
You don't have a rounding problem here in the first place. There is no (usual) way to round 0.1984... to the shown value of 0.12 . Rounding to two decimal places would give 0,20 .
By the way, your code is prone to division by zero error.
Answers
Check https://msdn.microsoft.com/en-us/library/dd301418.aspx to know about using ROUND.
Note you can only format up to 5 decimal places in a field in NAV, you can ream more about this property here...
https://msdn.microsoft.com/en-us/library/dd338809(v=nav.90).aspx
By the way, your code is prone to division by zero error.