The calculation used to reach the FOG Index on any work is:
1) Take a 100 word extract (ending at a full stop).
2) Count the number of sentences in this section.
3) Divide 100 by the number of sentences. This will show the average sentence length (ASL).
4) Count the number of long words (NLW), those that have 3 syllables or more.
5) Add the ASL and NLW then multiply by 0.4. This will equal the FOG Index.
To give you an idea of the FOG Index on different publications Reader’s Digest is 8 - 9, Time, The Wall Street Journal are 11, and The Bible, Mark Twain and Shakespeare are about 6.
Now whilst it would not be wise to measure every sentence you write by this calculation, or any like it, the formula can help give you an idea of how appropriate their writing is for your reader.
So here is the guideline to go by
7 - 8 = general populus
10 = average 15 year old
11 - 13 = college student
14 - 16 = university student
over 18 = too difficult for newspapers
And just in case you are wondering, from the excerpt I took of this article it rates at 9.2 on the index.