I think I'd prefer this to be an OBSERVATION.
The design appears to be a CLUSTER so that it can be inserted within the Exam findings family but it seems to me that this should be repeatable and confounding factors/state might be really useful in assessment of burns.
Not sure that specialising the current OBSERVATION.body_surface_area is a good idea. (Related note: Theoretically we DO need to specialise it for 'Adjusted' BSA if amputation etc)
We do need to consider the cross over between recording exam findings of burns - definitely Exam CLUSTER of some sort - with the recording of the total. Do we need to record the % per body area in the OBSERVATION or just in the findings CLUSTER? Suspect that we should record the burns finding in a repeatable Physical exam findings OBS rather than this repeatable Burns calculation.
For more pondering...