That's true enough. I had not actually forgotten about it, but when I did the calculation I was off by a factor of 10. (Still getting used to the fact that my new HP50g sometimes drops keystrokes if I enter too rapidly.) It didn't seem worth messing with.
Doing a linear interpolation between for 0.8ms to seek over 0.8% of the platter, and 8ms to seek over 50% of the platter, yields ~0.9ms for the seek. (And then we can add the 4ms for 1/2 rotation.) Whether using a linear interpolation is justified here is another matter.
However, let's not forget that there is *no* reason to think that the "Mixed Workload" phase is pure random read/write. The whole random seek thing is a separate side-question WRT the actual benchmark numbers I get. If the numbers *are* purely random access time for the drive, why do 4k records get me such incredibly dismal results, and 16 records get me near the peak sequential read/write rate of the drive? One would expect the random seek read/write rate to scale linearly with the record size.
BTW, thanks for the post. This has been a very interesting exercise.