question about rent payments

Hi - can anyone help me with this:

I want to rent a property advertised for $400 p/w.
I want to pay monthly, but there seems to be 2 ways to work out a monthly payment. Previous monthly rentals I have had have been worked out as the weekly rate x 52 / 12.
The RE agent however, works it out this way: weekly rate/7 x 365/ 12.
For a $400 p/w rent, this works out to be a $16 per month difference between the 2 methods. Is there a standard method of working out a monthly?

Thanks in advance