Calculating your average percentage and then converting to GPA isn't a great way to do it (although it would give an okay estimate). If you have a lot of variation in your grades, then it could be inaccurate though.
However, most applications ask international students (I'm from Canada, so I'm also "international" even though the Canadian and American system are similar) to report their grades in their own country's system and to NOT convert it.
But it's still useful if you want to estimate your US grade. I think the 2nd table in your Wikipedia link could be a good guide to do the conversion. So, for reference, this is how most US schools compute GPA:
1. For each course you took, convert the grade that is out of 30 into a letter grade. For example, a 30/30 would be an A+, and a 27/30 might be A-.
2. Then, for each letter grade, convert it into "Grade Points". In the US, usually, an "A" is worth 4.0 grade points, a B is 3.0 grade points, a C is 2.0 grade points, a D is 1.0 grade points. And then each + or - is 0.33 grade points (e.g. B+ is 3.33, A- is 3.67 etc.). Exception: Some US schools count A+ as 4.0 and some count it as 4.33.
3. You can now compute your GPA ("Grade Point Average") by taking the average of all your grade points from Step 2. To be more accurate, you will want to compute a weighted average. I'm not sure how it works in Italy, but in North America, there are a number of "credits" or "units" assigned to each course, where a "3 credit" course might be a course that is one semester long (13-16 weeks) and a "6 credit" course might be one that is two semesters long. Sometimes a course that is one semester but takes a lot of work will be worth "4 credits" etc. The proper way to compute a GPA is to weight each of your grade points by the number of credits.
If all your courses are equally weighted then the faster way is to convert your average out of 30 into a GPA range. If we are using a 4.33 scale (where A+ is worth 4.33) then 26.6/30 * 4.33 = 3.84. If you want to use a 4.0 scale (A+ is still worth 4.0) then 26.6/30*4.0 = 3.55. In any case, I think you are right when you say your average is about A- level (26.6/30 is more like A- than B+ in my opinion).
As for rankings, in North American schools, we also do not rank students. I don't mean an actual ranking but just an informal guess. For example, what I meant was is 106/110 an average grade or a really exceptional one, etc. And what would the distribution (e.g. standard deviation) be like? It sounds like you have answered this already and it looks like 106/110 is higher than the average! So that is good
So, to answer your last question, I think that yes, your grades are generally in the range where I would suggest people give the top schools a shot! Good luck