where to add fiscal regime code in customer master for Mexico sales organization?

For Mexico country, we have a requirement to add Fiscal regime code in customer master. Fiscal regime code is 3digit numerical code, Every customer has one code selected from list of given codes. Solution we are thinking here is to add this 3 digit code on attribute 8 field in customer master-> Extras-> Additional data-> Attribute 8 (Freely definable attribute). Please suggest if there is any better place to store this Fiscal regime code information in customer master.