As far as I remember, common sites are roughly twice as common as uncommon ones. If a rare site comes up, the site determination is rerolled and only if the second roll is also rare will the rare site appear. I don't know the percentages, but I suspect common sites are 50-60% of sites, uncommons at 25-30% and the rest rare.
The only way to hit a greater than 75% frequency for sites is to modify the map files. #features <percentage> is the command you are looking for.
I don't understand playing with site frequencies above 50. For me, 40 and 45 are pushing it. I like magic being scarce and research difficult so that you actually have to make decisions about what you want to do with your gems and where to take the research. 50 and above, magic is common as dirt and you're hard pressed to find a use for it all unless you're massively empowering mages to get extra paths for them.