With regards to PhD theses, what level of knowledge should one assume the reader to have? And, if needed, where should basic information be placed? With regards to PhD thesis that spans several disciplines, at what point does one assume that an article doesn't require explanation and it is  standard knowledge?

I have had this discussion with several academics and there doesn't appear to be consensus. One suggested a middle ground that I quite liked: Anything considered standard knowledge, should be placed into an appendix so as to not disturb the flow of the thesis but still somewhere handy enough for the reader to bush up on or explore. 

Is this a common technique? I've read so many theses that contain waffle and filler explaining (what I consider to be) the basic requirements of the field, however my option of "basic" is just that; An option. People have different degrees, life experiences, interests and so their options will no doubt differ regarding what is considered standard knowledge. 

So at what point is something consider "standard knowledge"? If a technique/paper/method is widely written about in master/Bachelor's course texts, journal papers and has a wiki page - does that qualify? For example; consider the most basis statistical techniques, such as chi squared test etc - does one really need to explain, in detail, what it is? Would a reference suffice or would a thesis begin to look ridiculously short? Perhaps placing it into an appendix?

Is there a general accepted rule of thumb that I am unaware of? Or is it truly a black art? I don't want to look lazy by not including material, but at the same time I don't want to waffle on.
Generally, the target audience of your doctoral dissertation is your doctoral dissertation committee. Write to their level.

In disciplines where your dissertation gets turned into a book (or series of articles), then you may consider writing at a lower level or with explanation for people outside of your specific subfield. You can certainly do this if you have the time or luxury to do so, but if you are short on time or funds, then it is perfectly acceptable to do what is noted in paragraph #1.