Faraway and Foreign

This book is highly recommended. By the way, I readily admit that blood and seawater have different saline concentrations. This fact was known to me even as I wrote the song. Here is a riddle: to what extent does this discrepancy affect the truth value of the song? The answer:

∆truth=±6.8

If you got a different (incorrect) answer then you may show your work for partial credit. Here is the song: