Get trade volume given entry price and stoploss using CheckOpenLong

Hi there,

Pretty standard question I'm sure but the answer seems to allude me.  I'm trying to calculate trade volume as a percentage of account equity given a specified entry price and stoploss.

#include<Trade\Trade.mqh>
#include <Expert\Money\MoneyFixedRisk.mqh>

CTrade trade;
CMoneyFixedRisk m_money;

double risk_percent = 2; // have also tried with 0.02 with same result in console.

void OnInit()
 {
  double entry_level = GetEntryLevel();
  double initial_stoploss = GetInitialStopLossLevel();
  double profit_target = GetProfitTarget(entry_level, initial_stoploss);
  m_money.Percent(risk_percent);
  double trade_volume = m_money.CheckOpenLong(entry_level, initial_stoploss);
  Print("entry: ", entry_level);
  Print("initial stoploss: ", initial_stoploss);
  Print("Vol: ", trade_volume);
 }

But trade_volume = 0.0.

How do I get the desired value for trade_volume please?

Thanks.