MULTIPLE BUILDINGS CONNECTED AS ONE.

I am trying to put together a water fixture inventory and calculations for an office complex that has three buildings built in different years that are attached as one building. The first building was built before 1994. The second building was built in 1998 and the third building was built in 2007. when filling out the fixture inventory do I seperate each building and do the calculations? And in the end put all three buildings calculations together. Or do I list each buildings fixtures and do a weighted flush rate. and then do the calculations off the weighted flush rates?