The working use of this concept for most investors (not endowments) is to look at the PE10 during 15 or 20 year periods - http://www.multpl.com/ - I did not find any constant range you can always count on. That is, the range from one 20 year period does not appear to me to be related to the next period.
Also, while one can point to extreme periods such as 1929, one can also point to 'overvalued' periods such as 2003-2007 and 2009-2011 when you would have 'lost out' on big gains if you had a light or non-existent equity allocation, not to mention you would not have known when the 'top' was that time.
Finally, I did not find in the paper where it details the 50/50 approach. Since it is the basis of comparison for the thesis, is it not meaningful to know whether the 50/50 allocation is rebalanced daily, weekly, annually, or bi-annually?
Bottom line is the 'relative value' of using PE10 does not appear to be constant in any working way for me - maybe I am just not smart enough to 'get it' - good thing I am a passive investor.

At the risk of sounding redundant...past performance is no indication of future results.