Flow for horizontal is the same, 60 liters per hour, per cm of screen. And more is better, like 100 liters per hour, per cm of screen.
Flow is even more important for horizontal, because when the algae grows, it grows up into the air, and stops the flow. So the flow must be enough to go over the algae. The algae can get 3 cm thick into the air. The flow must be very high to jump over this.
Horizontals are not very effective for this reason. The more they grow, the more they stop the water. So they must be 4 times the area, and 4 times the wattage, and high flow, to compensate. Also, tilting the scrubber more, will help the water get over the growth.
But after you make the scrubber 4 times bigger, and 4 times more wattage, and extra tilting, it is just as easy to make a vertical. So most people have stopped making horizontals.