How can I measure diagonal distance points?

You need to use Pythagoras' theorem.

d = Math.Sqrt(Math.Pow(end.x - start.x, 2) + Math.Pow(end.y - start.y, 2))

I think you're looking for the Euclidean distance formula.

In mathematics, the Euclidean distance or Euclidean metric is the "ordinary" distance between two points that one would measure with a ruler, and is given by the Pythagorean formula.

Well you might take a look at: https://en.wikipedia.org/wiki/Pythagorean_theorem