There's two kinds of perspective used in RPGs, which is something you have to keep in mind. Both are constructed differently. Novice artists use both mixed, don't know what they're doing, though.
First off, you have the perspective you show as "Correct". Orthographic projection, camera pointing 45° down. (Your measurements seem off, but I see you get the point)
There IS another one that's very commonly used and just as correct though. It's a variation of the cavalier perspective. Cavalier is where you take a normal front view (x & y are normal front view) and draw the z axis at a 45° angle out to the left or right to make it visible. The RPG variation draws the Z axis out to the top. The result is the "Traditional/Common RPG view" you reference. You also have to pick a foreshortening ratio for the Z axis (all values are valid, you just pick one and stick with it). Traditionally in RPGs, this value tends to be 1:2, meaning the lines running parallel to the z axis are drawn at 1/2 lenghth. Some go up to 1:1 though (which looks strangely elongated though).This is an equally valid oblique projection.
If my text isn't clear, I'll create some examples. The advantage of the second is that you can show the fronts of buildings normally, sprites are easier to construct, etc.