Is that where that spreadsheet originally came from - pianostreet.com?
It is one reference. We've noticed several inconsistencies within it. For example Beethoven's Moonlight Sonata and the Hammerklavier Sonata are both rated grade 8.
I'd only consider the relative difficulty of different pieces, and if they are both listed as grade 8 on this scale then refer to other references for more detail. There is no one universal standard, so you can't compare these numbers with other grading scales.
Other references are:
"The Pianist's Guide to Standard Teaching and Performance Literature" by Jane Magrath. This book ranks pieces from 1 to 10. These levels stop before you get to Bach's WTC or any but the easiest Beethoven Sontatas. The Season's are level 10 on this scale. Fur Elise is level 7
"Guide to Pianist's Repertoire" by Robert Hinson (was available through this link http://www.iupress.indiana.edu/iol/
) Rates The Seasons as M-D (medium difficult). Fur Elise is Int (intermediate).
"A Practical Guide to Solo Piano Music" by Trevor Barnard and Elizabeth Gutierrez. Picks up where the other books leave off - scale is from 1 to 4. 2 is the level of an undergraduate piano major, 3 is the level of an upper division piano major or graduate piano major, 4 is considered virtuoso. They break out The Seasons by piece, most or 1 or 2, September is a 3. Fur Elise is a level 1.
You can also look at the requirements for the exams, such as ABRSM or RCM. Some are available online, others you have to buy the current syllabus.
If you are taking exams, you should follow that exam syllabus.
Finally, some of the published repertoire series are good references. Here's the list of pieces in Keith Snell's Piano Repertoire series:http://www.kjos.com/pdf/brochures/snell_reper_index.pdf