The best way to calculate the requirements would be using a clamp meter. You can also try to calculate it by dividing the watts of your PSU with the volts the dc gives you but it's not the optimal way to determine how much you would use but how much your server is capable of.
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A cool thing about Dell servers is they have an online "Capacity Planner" app to show you wattage and other information regarding servers (and more). It may be worth looking on Dell's site to see if your server is listed. Unfortunately, I didn't see this device on the capacity planner.
Then you would need to calculate in the additional devices as stated above. You can get a kill-a-watt type device for cheap to check the devices as well.