Although I respect your right to mean what you like by drawdown I don't think your version of it here is as worthwhile as the one (I believe) Duritz has already employed - and I'll explain why.
Your figure is certainly your worst point of the period but when testing a system and not wanting to blow the bank you need to have a handle on the worst case scenario. If you had started the system at the end of the first month when in your example you were $500 in profit your maximum loss would be the difference between $500 positive and your lowest point. When testing a system you need to know what is the worst performance from a previous highest point to a following lowest point to cater for that dreaded run of outs - which may come right at the start of the betting cycle.
I'm going to stop now because as Iomaca pointed out, all this talk of runs of outs is beginning to make me feel tense.
KV