I converted the 400L/H = 105.669 gallons per hour
You say it's a 2ft tank, I assume it's a 15 gallon or 20 gallon tall.
I'd say there should be just enough flow, a bit on the weak side, but enough. See how well the flow does, if you have tiny dust/debris particles in the water they are great to see how the current of water travels throughout the tank. Some like to see the plant leaves "blow/wave in the wind/current" so that can be an indicator. Before trying a powerhead, play with the spray bar angling to see if changing it's aim makes the flow throughout the tank any better.
It does vary, since if you do have a lot of plant mass, it can block the water flow to the other plants, so then you might want a additional powerhead (circulation pump preferred, more wider dispersing flow).
After some time, if certain types of algae (thread, staghorn?) are growing on the plants, I believe that may be a sign of poor water flow in that area.