Do you have smart meters on the building and does the utility keep 15 minute interval usage data per meter? If so, and you can get that for all units, all you need to do is sum the loads of each unit together if they use the same time intervals (assuming the car charges are coming off of each units panelboard) and check the total KW being drawn. This is like what you do in 220.87 to do a load calc based on usage instead of overly conservative calculations.
Not sure if the poco would turn over each individual's power history to a condo board, so may need to have each owner give permission to give it out. If the poco doesn't have this data, then it isn't a solution, but you could possibly put some power recording device on the service to measure it. Someone needs to read the data monthly or quarterly to see what the loads are. Continuous loads are a bit tougher to measure as 220.87 data asks for 15 minute intervals. You would need to find the worst case 12 consecutive intervals to get a continuous number. Probably doable with an Excel file, and my poco provides the data in excel format directly.
I also wonder about your assumptions in post 1. A 90A breaker to a unit doesn't mean their load calc is 90 amps. How did you determine it was a 600A service? Did you see multiple 200A breakers in meter stacks and add them up, or is there a single 600A main, or did you examine the service conductor size to see what it is. If there are multiple service disconnects, they are allowed to sum to larger than the calculated service and most likely are. The service conductor must be sized to the calculated load, not the sum of the main breakers when there is more than one.
