Hi Wes,
Actually you don't need to script anything for that. It is already implemented in the object Palette of the decimal field.
Open: Object Palette ► Field
As your Type of field should be Decimal Field you should be able to see under Pattern ► Limit Leading Digits & Limit Trailing Digits
"Leading Digits" means numbers before the comma
"Trailing Digits" means numbers after the comma
If you are looking for some kinda currency or percentage, you can also use the Pattern num.currency{} OR num.percent{}
