Your example is great and almost how I calculate also.  EXCEPT, you are using different wattage amounts when I use the same wattage and just extrapolate the hashing and asset cost difference.  When calculating based on lower total watts used, I can definitely see the underclocking stance.  I make the assumption that miners would want to use as much of their circuit for hashing as possible.  So if I have 4,000 watts, what does the asset cost difference between under/over clocking and recoup from the additional hashrate.

Although it's all just mental masturbation as it's doubtful right now these will ever ROI in btc terms.  Fun hobby that keeps me off the streets