tab order of help button in dialogs

When I place the help button in dialogs like here:

What tab order should I use? It's obvious that the content panel uses tabindex values that correspond to the reading order which is supported by the optical layout (which may indeed be questionable in this very case). My question is only about the relation of the order of content, Help, OK, Cancel.

The reading order clearly suggests content, Help, OK, Cancel. Whereas IMHO the fill-in order suggests content, OK, Cancel, Help because you change input controls via tab key, and after completing the form, it would make sense to get to the OK button. To me, this looks like a dilemma, but I have only a limited knowledge of UX.

What is the preferable option from the usability POV?