Fix lot size calculation and help me understand your fix

I have a function in my EA that returns the lot size based on a provided entry and sl level.  The risk percentage of equity is also provided.  Sometimes the EA allocates too much risk and some times too little.  I can provide examples of each.  

Please help me to get the function to

1. allocate risk correctly no matter the instrument and or symbol including stock cfds, currencies, futures, commodities.

2. show an error indicating that the minimal lot size exceeds my risk allocation if necessary.

The code should:

1. prioritise readability

2. contain clear comments.

Please be willing to answer any questions I have to understand the code.

CSymbolInfo m_symbol;
CMoneyFixedRisk m_money;
CTrade my_trade;

risk_percent = 2;

double GetTradeVolume(string symbol)
  {
   double return_value = 0;
   ENUM_TIMEFRAMES timeframe = (ENUM_TIMEFRAMES)GlobalVariableGet("g" + symbol + "_timeframe");
   double entry = GlobalVariableGet("g" + symbol + "_entry_level");
   double sl = GlobalVariableGet("g" + symbol + "_sl");
   
   m_symbol.Name(symbol);
   m_money.Init(GetPointer(m_symbol), timeframe, (double)SymbolInfoDouble(symbol, SYMBOL_POINT));
   m_money.Percent(risk_percent);
   
   if(isLong)
      return_value =  m_money.CheckOpenLong(entry, sl);
   if(isShort)
      return_value =  m_money.CheckOpenShort(entry, sl);

   return_value = NormalizeDouble(return_value, 2);
   
   return return_value;
  }