You mentioned measuring distnce between elements but I can't seem to find it?
Generally it would be great to have a free measurement tool that snaps to pixels / element bounds / canvas bounds etc, that will let developers do their own measuring, since you never really know how they intend to implement it.
Thanks and great job and project!
Select the first element by clicking on it and then hold down the shift key while selecting the second element by clicking on it. You should see a black box containing the width and height between the two elements.
Hope that helps!
Still I'de vote for a free measurement tool as well.