Linwood, that is helpful information. The "odd duck" in the mix seems to be the Lexar 800x, for which (and I think someone else pointed this out) the read speed is stated to be (according to B&H website) 120 MB/s (consistent with the formula stated by you) and the write speed of which is stated to be 45 MB/s. The stated read and write speeds of the Sandisc cards are, I think, the same and consistent with the .15 forumla (e.g. the 600x cards state read and write speeds of 90 MB/s. Based on this information I think I will scratch the Lexar 800x card from consideration due to the relatively slow write time.
Thanks for the post. I think I may have learned something.