Documenting per building in the case of central purchasing

On our campus all cleaning supplies are purchased centrally by our housekeeping vendor. We are struggling with how to assign cost for purchases in an individual building as we don't track purchases per building or which items are used in each building. I believe we need to document a "worst case scenario" but I'm looking for help on what we would need to supply. Breaking down the cost per square foot per building? Cost per FTE per building? Likely noncompliant purchases per space as a percentage of sq ft per building? (e.g. 5% of the building is bathrooms and showers where we use non-compliant products - even if that was 100%, we would still comply)