Series wound DC motor at .75 HP equals .75 x 746 w for 559.5 w at 10 volts thats 60 Amps rated at 1x. Series motors can draw up to 6x, but my 1100 has been monitored at 3.5 x So 60 x 3.5 is 210 A for 1 sec, amperage drops considerably and settles around 100 A nominal for 5 seconds at a time.
So using safe rule of thump
With a good battery, good cables and connections, good charge system and at 65 F ambient 100 A is a good number.
But all hell breaks lose when any of the conditions are not met.
By the way I have measured the amps on a cold start on 80 1100 in good tune with all conditions met and it is between 95 and 105 amps.