Alternator Sizing - Amp requirements

If you have not, be certain you deal with the "bulkhead connector/ ammeter bypass" if you have not done so

That's one thing I don't have to worry about. I've already welded up the firewall and have done away with the stock bulkhead connector. The stock gauge cluster is LONG gone and I will not be running an ammeter at all. There is not a factory wire left in the car and I will be wiring it from scratch.

I'm just struggling to figure out how to size the alternator for what I need. Say I add up the total amp draw of everything I'm running. Do I then to to ensure the alternator can produce that much power at idle??? I know "perfect world" scenarios and "real world" scenarios are different. I'm just trying to get a general idea on what will do the job.