How does one calculate an optional e?

By figuring out the difference in your Dewey numbers, that's how!  Thanks to Renee of the inner librarian for bringing this johnson to my attention.  

Here is with an E:

Kristie's Dewey Decimal Section:
103 Dictionaries & encyclopedias
Kristie = 1899095 = 189+909+5 = 1103

Class:
100 Philosophy & Psychology

Contains:
Books on metaphysics, logic, ethics and philosophy.

What it says about you:
You're a careful thinker, but your life can be complicated and hard for others to understand at times. You try to explain things and strive to express yourself.

Find your Dewey Decimal Section at Spacefem.com

And here it is without: 

Kristi's Dewey Decimal Section:
098 Prohibited works, forgeries & hoaxes
Kristi = 189909 = 189+909 = 1098

Class:
000 Computer Science, Information & General Works

Contains:
Encyclopedias, magazines, journals and books with quotations.

What it says about you:
You are very informative and up to date. You're working on living in the here and now, not the past. You go through a lot of changes. When you make a decision you can be very sure of yourself, maybe even stubborn, but your friends appreciate your honesty and resolve.

Find your Dewey Decimal Section at Spacefem.com

Please note: I find both of these to be extremely accurate.  This further suggests that my E is actually optional.  Score!  

Also note:  The incorrect spelling of my name (Kristi) is the Dewey number for forgeries & hoaxes.  Figure that.