In Chinese, every character is monosyllabic and Mandarin has a total of about ~1200 licit syllables including tone (about 400 if you don't count tone). This means that if I take any Chinese character and randomly choose a Chinese syllable, there is roughly a 1 in 1200 chance that that syllable will be the correct pronunciation.
Now let's say I do the same thing with Japanese. I take any random Kanji character and I randomly choose a full phonological form that can be represented by a single character. What's the probability that the pronunciation will be correct? This is a much more complicated question than in Mandarin since a whole multi-syllabic word can be represented by a single character, but like any language, there is a finite number of licit phonological forms in Japanese. Are there any accurate estimates of this number?
Edit: To illustrate:
If I pick the character 鏡 then the phonemic form can be kagami. If I select 鵜, the phonemic form can be u. For 八, hati. I'm not particularly concerned with the characters being used in a multi-character word, my primary concern is solely with individual characters.
So if I pick the character 鏡 and then I randomly select from all of the full phonological forms that can be represented by a single character, what is the probability that I will correctly select kagami?
In Mandarin, because there are only 1200 syllables and characters are always monosyllabic, if I pick a character with one possible pronunciation I have a 1 in 1200 chance of getting it correct. Some polyphonic syllables may have multiple possible pronunciations so I could have a 2 in 1200 or 3 in 1200 chance of picking it correctly.
In Japanese, a character can represent a sequence of multiple syllables, so what's the chance that out of the list of all possible phonological forms I'll pick the one that consists of the correct sequence of syllables?
Edit 2: Overly detailed reason for the question:
I'm researching cross-script differences in reading processes between English and Chinese; here's a highly simplified explanation.
In English, readers mostly compute the sound of a word based on the letters and use that computed phonological form to pick the semantic information behind the word. Visual information play a smaller role, but it helps with things like disambiguating homophones (e.g. "flower" and "flour"). A reader sees "flower", computes /ˈflɑʊ.ə˞/, and activates the meaning ❀, maybe with some help from visual information.
In Chinese, readers mostly retrieve semantic information directly from the visual form. Phonological information plays a role, but it doesn't seem to be the primary route of activation. A reader sees "花" and directly activates ❀.
Like I said, this is a super simplified explanation, but it covers the main gist.
I want to approach this difference as having to do with the "usefulness" of the different routes. In English, the visual information route isn't "useful" because English speakers don't make direct associations between visual forms and meaning.
In Chinese, one possibility is that the phonological route isn't "useful" because that information is very non-specific. A Chinese speaker can read "书" and know what it means with no context because the meaning is directly associated with the visual character. However, if there wasn't that direct association and the character simply activated "shū", it would be difficult to retrieve a specific meaning with no context because that phonological form can represent several dozen morphemes.
I'm trying to figure out if Japanese could be examined as a language kind of "in between" these two. Chinese cannot use a purely phonological route to get from any one of tens of thousands of characters to any one of hundreds of thousands of meanings, because you would have to connect them through a thousand or so phonological forms, forming a serious cognitive bottleneck. If Japanese can get from any one of tens of thousands of characters to any one of hundreds of thousands of meanings by going through several thousand possible phonological forms, Japanese could potentially have better success with the phonological route.
The question basically comes down to this: is the bottleneck from form to meaning through phonology in Japanese nearly as bad as it is in Chinese?