Deleted this message, have recieved email from White labs and confirmed there formula is same as Wyeast just done in another fashion, the -1 was meant as -1.0 and not as subtracting 1 point. My mistake. Here is the reply from White labs.
That is just the formula simplified, your probably doing it automatically by not using the 1 in your formula, for example instead of 1.060-1.010, your doing 60-10/60. But you can see how 1.060-1.010/1.060 won't work.