Maximum lot allowed by margin (OrderCalcMargin)

double MaxLotByMargin(string symbol, ENUM_ORDER_TYPE type, double marginBudget)
{
   double price = (type == ORDER_TYPE_BUY)
                ? SymbolInfoDouble(symbol, SYMBOL_ASK)
                : SymbolInfoDouble(symbol, SYMBOL_BID);

   double marginOneLot = 0.0;
   if(!OrderCalcMargin(type, symbol, 1.0, price, marginOneLot) || marginOneLot <= 0.0)
      return 0.0; // symbole non synchronisé ou calcul impossible

   return NormalizeLot(symbol, marginBudget / marginOneLot);
}

// Exemple : ne jamais engager plus de 25% de la marge libre
double budget = AccountInfoDouble(ACCOUNT_MARGIN_FREE) * 0.25;
double maxLot = MaxLotByMargin(_Symbol, ORDER_TYPE_BUY, budget);