What is Flat rate?
An interest convention that charges interest on the original principal for the full term, whatever has already been repaid.
It always understates the true cost of borrowing, and the understatement grows with the term. Converting it to a reducing-balance equivalent needs a numerical solve, not a rule of thumb.
Not to be confused with Reducing balance
Under a reducing-balance rate you stop paying interest on money you have repaid. Under a flat rate you do not.