**How** harmful is a hard disk spin cycle?

I am not aware of any studies on the subject, but I do know what the SMART data tells me:

For one particular drive (a WD Scorpio Blue 2.5") a start-stop count of ~200,000 or a load-cycle count of ~600,000 corresponds to SMART value 0 (i.e. the disk is at the end of its life according to SMART). (This is a laptop drive, they are made to handle a larger number of spindowns than desktop drives are.)

As these values come from the manufacturer, I assume they represent the manufacturer's best guesstimate for what their drives can handle. Lacking independent data, I'd be inclined to think that the manufacturer's guess is probably better than mine, so you could probably do worse than using those numbers in calculating the X.

The google study does mention the effect of power-on cycles:

Power Cycles. The power cycles indicator counts the number of times a drive is powered up and down. In a server-class deployment, in which drives are powered continuously, we do not expect to reach high enough power cycle counts to see any effects on failure rates. Our results find that for drives aged up to two years, this is true, there is no significant correlation between fail- ures and high power cycles count. But for drives 3 years and older, higher power cycle counts can increase the absolute failure rate by over 2%. We believe this is due more to our population mix than to aging effects. More- over, this correlation could be the effect (not the cause) of troubled machines that require many repair iterations and thus many power cycles to be fixed.