Given the tools at hand if this was a critical shot I'd set the meter to 400, walk into the shade in the building to take a reading with my hand pointed at the camera.
At the chair I'd point my hand directly at the sun and then take a reading while pointing the meter directly at my hand, disregarding the camera position.
I'd average those two readings and do the math to "open up" 1-stop to get my camera setting.
Other than Bill's EI adjustment, in this situation I think Bill and I would find very close to the same settings.
I'd develop in DD-X per Ilford's instructions for HP5 at 400.