Rate of cathodic reaction vs Tafel slope

One of my reference engineering books says that for the cathodic protection process (in seawater): 


Al $\rightarrow$ Al$^{3+}$ + $3e^-$ (anode, setting the protection potential at about -900 mV SCE)


2H$\_2$O + 2$e^- \ \rightarrow$ H$\_2$ + 2OH$^-$


Book says that the rate of hydrogen evolution grows roughly one order of magnitude every -120mV of over-polarization (more negative potential). 
I found online that the Tafel slope for the hydrogen evolution process is roughly $\beta$=200 mV/dec in seawater (should be $\beta$=120mV for uni-molecular mechanisms).
Following this reasoning, I get (using the Tafel equation):


ln$\frac{i\*}{i}$=ln10=-$\frac{1}{\beta} \left ( \eta^\*-\eta \right )$


therefore the rate should rise one order of magnitude every $\Delta\eta$=460 mV ($\beta=200mV$, and $\Delta\eta$=276 mV if $\beta=120 mV$).
Does anybody see any mistake in my reasoning? If this is correct either my Tafel constant is off (should be about 55 mV) or the book has a typo... 


Thanks in advance.


