When you say purlins, do you mean beams supporting the rafters that are parallel to the ridge and between the ridge and eave?
Great. So with that in mind, what exactly is the question? The bulk of the roof load appears to be carried by these purlins.
Based on your description, 12.5% goes to each exterior wall (25% total) and 37.5% to each purlin (75% total).
The joists are typically perpendicular to the ridge (parallel to the rafters). If they are parallel, then you are correct, but that would be atypical.