I could write for hours on the complexities of decking structure design, have a look here
There are post, beam, joist and decking span calcs that should be adhered to.
The main reason is if you don’t, whilst your completed project may look ok, in 6 months it will simply twist, buckle, snap screws and become unsafe, and you would have wasted your money.
In short 400 mm joist centres are only a small part of decking design. These numbers are there for a reason, 400 centres with pairs of countersunk holes for bugle head stainless screws are essential to prevent the timber from twisting, at 600 it will twist alright, snap the screw heads off too. The numbers are also integral to the loading the decking can withstand safely, 1.5kn is what is required and can only be achieved at 400 centres.
There are several pages on this site with advice for decking construction, if you are in need of advice, ask your questions and I am sure they will be answered…