I'm unsure what failed logic you are trying to use but MATHS is not the issue here, its a missing build piece that doesn't allow roof ridges to connect with a right angle roof, the degrees of the item is irrelevant as shown here the full 26 right angle roof being snug
https://steamcommunity.com/sharedfiles/filedetails/?id=2626846762

You are therefore forced to construct a roof without the ridges if you want to do anything other than a straight line as outlined in the 3 photos