Instructor is adamant that the outside dimensions of the area defined by the spreaders disregarding the space inbetween is the area to use for the calculations thus giving a floor load of 444kg/m2
I doubt that the aircraft OEM would have a similar view ... Effectively, we are concerned with localised penetrating or crushing stress loads so the actual contact area is that which counts. To take your trainer's view to the extreme, I presume that he would come up with a similar view if the shoring were, say, 4 sheets at each corner with negligible contact under the box but still with a few square feet extending outwards ? Basically doesn't hold water ...
Spreaders assumed to be suitable...
Therein arises my question .. there is suitable and not quite so suitable.
Reference to the OEM Weight and Balance or Loading Manuals would be the first port of call to resolve what the OEM might have desired. If these documents don't exist or don't resolve the question, then one flicks an email to the OEM Tech Support folk with the question ...
I might disagree a little with underfire's comments. Running load, I suggest, is more concerned with fuselage bending capability rather than dynamic loading. The OEM will have designed the thing to suit the design G-loads which should cover most of the likely to be encountered in-service loading.