Graph of the SFST's HGN = Horizontal Gaze Nystagmus test accuracy
This is a graph of the never-before-available-to-the-public data in the NHTSA's 1998 San Diego validation study. Each point represents one driver in the study: OLS score on the x-axis, BAC on the y-axis. If you know a driver's HGN score, do you really know their BAC?

R^2 = 43%. Only 43% of the variation in BAC can be explained by variation in HGN score. (R^2 is the "Coefficient of Determination")

When innocent drivers failed the HGN test, police in this study violated standardized FST interpretation guidelines about half the time, and simply ignored failed HGN results. Does the officer in your case know how to do that?