Correct oil temperature is dependent on the size of the food being fried. Larger items require a lower temp so they will cook through and brown at the same time. Smaller items should cook at a higher temp for the same reason.
A thin piece of seaweed should be cooked hot and fast as you said. I would take GB's suggestion and get a thermometer. I'd aim for a temperature near 375-390 F.
Your stove knob won't work as a reliable indicator. The size and shape of the pan, the amount of oil and the power of the burner are all variables that make it unreliable.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan