Has anyone actually made a legitimate alphabet key thingy?

[Mauro]

I think it is more useful to consider how a writing system scores in two respects.  For a person with basic command of the language,
(a) reading determinism: how reliably can they correctly pronounce an unfamiliar written word.
(b) writing determinism: how reliably can they write down the correct spelling of an unfamiliar spoken word.

Italian gets maybe 90% score on (a); not 100% because stress is phonemic but is not marked, and has two "e" sounds and two "o" sounds that are not distinguished in writing.  ("botte" can have either "o", meaning either "hits" or "barrel"; and "pesca" may be either "peach" or "fishing" depending on the "e").  Offhand I think that it has 99.9% on (b), but I may be wrong.

It's always a pleasure to read from you about linguistic matters (I also liked a lot your post You are not allowed to view links. Register or Login to view.).

I can confirm Italian surely does not get 100% on score (a). Beyond "e" and "o", "s" too represents two different sounds ([s,z]) and so does "z" ([ts,dz]), and approximants [j] and [w] reuse the same symbols as [i] and [u] (even if [kw] has its own symbol: "qu"). There are also cases where one cannot be sure if an "i" must be pronounced or not, and rarer cases involving "gl", so it's actually difficult to correctly pronounce Italian from orthography only. Years ago I wrote a program to check how much phonetical Italian is and, surprisingly, only about ~31% of word tokens in an Italian can be exactly pronounced relying only on the orthography. This does not matter to an Italian, in most cases a word will be understood both, say, with an [s] or a [z], but can be confusing for a foreigner. My wife had a colleague coming from Kashmir who asked her how "s" must be pronounced, when it's [s] and when it's [z], and my wife could not help her, indeed she hadn't ever realized before that "s" stands for two different sounds.

On score (b) we'll get a high percentage, but less than 99.9%. Ie. in a word such as "scienza" (science) it's by no means clear why we need an 'i', which is unneeded and is not pronounced by almost anybody (there are historical reasons, of course). And I vouch for a spelling reform of 'scuola' and 'cuore' (school and heart) to 'squola' and 'quore', which is how they're actually pronounced by almost everybody, with [wɔ] instead of the diphtong [u.o]. Teachers in primary school would live a better life (not to speak of the kids!).