Expert Advisor Programming by Andrew R. Young

Advanced Order Placement
If you must use a large stop loss, or none at all, a fixed lot size would probably be more beneficial.
We need to be able to choose between calculating the lot size or using a fixed lot size. Let's use an
external boolean variable called DynamicLotSize to turn our lot size calculation on and off:
// External variables
extern bool DynamicLotSize = true;
extern double EquityPercent = 2;
extern double FixedLotSize = 0.1;
// Start function
if(DynamicLotSize == true)
{
double RiskAmount = AccountEquity() * (EquityPercent / 100);
double TickValue = MarketInfo(Symbol(),MODE_TICKVALUE);
if(Digits == 3 || Digits == 5) TickValue *= 10;
double CalcLots = (RiskAmount / StopLoss) / TickValue;
double LotSize = CalcLots;
}
else LotSize = FixedLotSize;
If DynamicLotSize is set to true, we will calculate the lot size based on the stop loss, and assign that
value to the LotSize variable. If DynamicLotSize is false, we simply assign the value of
FixedLotSize to LotSize. The LotSize variable will be passed to the OrderSend() function as the
lot size for the order.
Verifying Lot Size
Just like the stop loss, take profit and pending order prices, the lot size should also be verified to
make sure it is acceptable to your broker. This means that your lot size should not be too large or too
small, and it should not be specified in micro lots (0.01) if your broker doesn't support those. You
should also normalize your lot size to the appropriate decimal place.
Let's check the minimum and maximum lot size first. The MarketInfo() function, using the
MODE_MINLOT and MODE_MAXLOT parameters, will be used to compare the current lot size to the
minimum and maximum lot size. If the lot size is not valid, it will automatically be resized to the
minimum or maximum.
if(LotSize < MarketInfo(Symbol(),MODE_MINLOT))
{
LotSize = MarketInfo(Symbol(),MODE_MINLOT);
}
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