This simple script determines whether a new trade is viable according to the current level of portfolio risk. It does NOT take into account any strong positive or negative correlations that exist with the existing trades and only uses the AccountBalance() and NOT any draw down amount in equity.

checkPortfolioRisk.mq4

extern int  maxPortfolioRisk = 30; // maximum amount at risk (0 = nothing; 100 = all)

// this function returns the profit/loss if the stop is hit
double profitAtStop( double open, double stop, double lots, bool isLong, string sym = "" ) {
    if ( sym == "" ) { sym = Symbol(); }
    double result = lots * MarketInfo( sym, MODE_TICKVALUE ) / MarketInfo( sym, MODE_TICKSIZE );
    if ( isLong ) {
        return( N( ( stop - open ) * result, 2 ) );
    } else {
        return( N( ( open - stop ) * result, 2 ) );
    }
}

// this function will calculate the total profit/loss if all current trailing stops were hit
double getStopPL() {
    int tot = OrdersTotal();
	double result = 0;
	for( int i = 0; i < tot; i++ ) {
		if ( OrderSelect( i, SELECT_BY_POS, MODE_TRADES) ) {
			if ( OrderType() == OP_BUY ) {
				result += profitAtStop( OrderOpenPrice(), OrderStopLoss(), OrderLots(), true, OrderSymbol() );
			}
			if ( OrderType() == OP_SELL ) {
				result += profitAtStop( OrderOpenPrice(), OrderStopLoss(), OrderLots(), false, OrderSymbol() );
			}
		}
	}
	return( result );
}

// placed somewhere within your system, once entry gates have established that there
// is going to be a trade

double newTradeRisk = profitAtStop( openPrice, iniStopLossPrice, qty, isLong, symbol ); // data needs to be entered here relevant to the trade
double stopPL = getStopPL() - newTradeRisk; // where newTradeRisk is the amount to be risked with current trade
if ( stopPL / AccountBalance() > -( maxPortfolioRisk / 100 ) ) {
    // ...process trade...
}