I'm working on a dictionary tool that provides, as an auto-assist, transcription of Chinese characters into corresponding (Modern Standard Mandarin) pinyin+tones on a naive, character-per-character basis without lexical parsing (the tool is intended for indexing of new terms/phrases, lexical parsing is overkill).
In order to make the auto-assist feature as useful as possible, I would like the assisted-transcription operation to choose the most frequent reading  of any individual character as the preferred default in an effort to reduce the amount of time users will spend correcting the inevitable  mistakes.
Python's pinyin package, based on Mandarin.dat, seems to provide reasonably good performance in that regard; but I'm curious whether there's a standard (issued by a language authority in PRC or Taiwan) list of such frequencies available, and/or whether any list exists that includes the other possible readings, in decreasing order of frequency, by way of providing a helpful dropdown list of alternates in cases where the preferred reading doesn't happen to be correct.
 i.e., assuming a large and representative set of textual data, if a character has multiple possible readings, the pronunciation most often assigned the character based on more advanced lexical analysis of the function of the character within a larger semantic block. For example, if within the set there are three observed instances of 为[wei4]什么 and one observed instance of 作为[wei2], choose "wei4" as the default transcription option for 为.
 I'm aware of characters such as 调 for which alternate readings (in this case diao4/tiao2) are equally likely, so in these cases such "assistance" will likely never have an accuracy rate better than 50%.