How accurate, I don't know. As far as I know there's not much information on how suitability is calculated.
I don't know how familiar you are with chapter 4 of the manual (which is a tutorial of sorts) but the section about leaders gives some useful pointers to at least get started.
Personally, I like to check in the manual the Skills required by the various tasks of the Council to staff, and look how the candidates fare from this point of view.
Bearing in mind that Skills in the same category and the relevant attribute (such as Intelligence, I don't have the exact term handy) can provide a bonus to Skill rolls, so I take that in consideration too.
In the end, I look at the Capability Level to assess if the leader is destined to improve quicker rather than slower or not, and then I have some idea of suitable they are for the job.
For instance, my Models Design Council director was a merchant and had no Technician skill which would have been ideal, but he had good Improvisation and above all Intelligence, and his Cap. Lvl is III. He was much better than the alternative. It's only been about half a dozen turns or so, but he has already raised his Technician skill to 6 by spending XP on it and provides +11% skill bonus to his council's tasks performance (and about +50% more but that's because he likes me so very much).
Supreme Command and governorships are a tad more complicated because they require Skills from different categories moreso than other positions do, and I must say the Skills needed do not seem to me very well consolidated in one place to check in the manual, but the principle remains.
But at the end of the day, I often find out that I have to make a sub-optimal choice because of politics.
< Message edited by Jdane -- 6/16/2020 7:20:07 PM >