Only in Theorie can you calculate how much the ballast of an HMI draws, and yes, it ALWAYS spikes when starting up, even though the brand new electronic ballasts do that a bit less than the previous generations of electronic ballasts.
The reason any calculation will be somewhat or a lot of in the Real World is: there are too many unknown factors.
How much cable do you have on the floor before reaching the ballast.
What kind a cable ?? (ALWAYS continue as long as possible with the most powerful cables, before splitting them up !!)
How old are the cables ?
How well maintained are they ?
Is it freezing or rather warm/hot on location ???
European Power is different from North America, so maybe it is different for you over there (although I did not ran a mock in Montreal, Quebec, when shooting there…), but I always TRY to stay at a 2/3 load of the full capacity.
Of course, sometimes that is not possible, money wise.
The Link Guy Holt provided is great !!
many Greetings from Amsterdam !!