The amount needed to daytrade ES or NQ (for example) is daytrade margin required by broker and amount of maximum drawdown of your system (maybe multiplied by 2). So if my system has maximum drawdown 800-1000 for one contract - I will count it as 2000. Then I can add broker margin requirement and that is. For AMP I need 400 (ES) + 2000 = 2400, for Tradestation I need 6600 (50% of initial) + 2000 = 8600. With 8600 dollars you can daytrade one contract with Tradestation or 3 contracts at AMP. But you have to have reliable and backtested system and follow the rules to avoid higher drawdown. You do not need 10000 or even more for one e-mini contract.
That's a great rule of thumb. It also makes more sense to me (since the maximum loss I'm willing to take in a day is $500 - to have about $1400 per contract position with AMP's $400 margin per position).
Thanks!