The standard pre-dilutions for the Coatest® SP FVIII assay dont make sense to me. For example, to get 100%, I dilute 100 ml plasma with 50 ml buffer, but to get my 50% standard, I dilute 100 ml plasma with 200 ml buffer. Why?

The key is that the plasma that does not have a pre-dilution corresponds to 150%, not 100%. One way to look at it is that since 100 is 67% of 150, then the plasma must be diluted 67%, which corresponds to 100 ml plasma with 50 ml buffer (100 plasma / 150 total volume = 0.67). This process can be followed for all of the standard pre-dilutions.