As I am unlikely to have this full I am tending towards putting a longitudinal divider and turn it into two tanks 400 wide by 500 long and 750 high.
The purpose of the baffle is to act as internal bulkheads to support the span of the long side and reduce the wall thickness of the tank. A full tank of fluid on a moving boat can make some serious forces.
With a volume of 300 liters that would mean the volume if filled with water
would run about 300 kg. You can see as you get bigger the baffles are meant as structural not just to reduce the sloshing. Even if you don't plan on filling it you need to make sure you could. Overfilling the tank would then be a time bomb waiting to happen.
tank is stainless steel
The baffles are solid across but there are notches in the baffle where it is attached to the sides and the bottom to allow the tank to be level between them. That gives a good attachment to hold the long sides from pushing outward as well as maintain the rectangular shape of the tank under dynamic loads.
So the rule
of thumb is you need to minimize the deflection of the faces of the tank based on the tank volume and the material of the tank walls. Long expanses tend to push out and worse yet the tank can tend to "rack" and break the seams at the edges. Reducing the sloshing also reduces fluid motion and noise
that also reduces the dynamic moment when pounding through surf. 300 kg bouncing around is a lot of forces. The seams actually stress and release and can fatigue.
You also want to secure each edge of the tank in two directions and at least two or more places so the hull
will support the tank motion. The idea is the tank should be strong enough to freely support itself when full but also have no place to go as you fly through the water. Repeated pounding will fatigue the metal and probably worst at seams.
It would not hurt to actually have the tank engineered and avoid just a rule of thumb. It's just a box but it has to handle a lot of dynamic load.