Bulk Items units from area to volume

In the PTC documentation for bulk items, they use a relation to build a value with the area of a selected surface (https://support.ptc.com/help/creo/creo_pma/usascii/index.html#page/assembly%2Fasm%2Fasm_four_sub%2FTo_Define_Bulk_Model_Properties.html%23). The question is, that within Creo, the units for this process seems to be a 2d area (sq inch). However, when people buy or apply bulk items (like paint), they buy and measure in volume (cubic inch, gallons, etc.). What do other people end up doing for the BOM when dealing with Bulk Items and units?