150 × CurrentHP ÷ MaxHP
Therefore when your Current HP is 3/5 or 0.6 that of your Max HP, Water Spout is the same power as Surf because:
[n = Max HP]
150 x 0.6n ÷ n = 90
n ÷ n = 1
150 x 0.6 ÷ 1 = 90
Power doubles if opponent's HP is less than 50%.
So its' power has no relevance to when Surf's power is greater than Water Spout's because they change depending on different variables: Water Spout = Your HP and Brine = the opponets's HP.