Particular Column Value modification allowance

Hi,

I am working on NAV 2013 R2 Database.

I want that No one can change the Document (record) if this Document (record) carries a TRUE value in a Boolean field. Only those can change who has proper rights in USER SETUP TABLE (ID 91).

But at the same time I have to give a provision to the end user to change the Posting in same Record if required, but can not change other field level records in any circumstances.

To achieve this I have added the following code under the Table/Page Triggers.....

TABLE----
OnModify()
IF Approved AND (xRec."Posting Date" = Rec."Posting Date") THEN BEGIN
  UserSetup.GET(USERID);
  UserSetup.TESTFIELD(Authorized,TRUE);
END;

PAGE----
Posting Date - OnValidate()
CurrPage.UPDATE(TRUE);

This is working absolutely fine. Is there any other way I can manage the same ?
Kindly inform.