Calculating the Required Margin

I am trying to calculate the Required Margin for a certain trade, this is a sample bot of what I am trying to do:

using System;
using cAlgo.API;

namespace cAlgo.Robots
{
    [Robot(TimeZone = TimeZones.UTC, AccessRights = AccessRights.None)]
    public class RequiredMarginBot : Robot
    {
        protected override void OnBar()
        {
            double volume = 10000d;
            double estimatedRequiredMargin = (volume/Account.PreciseLeverage) * (Symbol.Bid); 

            // if Account Currency matches the Base Currency
            if (Account.Currency == SymbolName.Substring(0, 3))
            {
                Print("Account Currency Matches Base");
            }
            // if Account Currency matches the Quote Currency
            else if (Account.Currency == SymbolName.Substring(3, 3))
            {
                Print("Account Currency Matches Quote");
            }
            else
            {
                throw new InvalidOperationException();
            }

            double actualRequiredMarginBefore = Account.FreeMargin;

            ExecuteMarketOrder(TradeType.Sell, Symbol.Name, volume);

            double actualRequiredMarginAfter = Account.FreeMargin;

            Print("Account Currency:\t" + Account.Currency);
            Print("Symbol:\t\t\t" + SymbolName);
            Print("Estimated Required Margin: " + estimatedRequiredMargin.ToString("0.00"));
            Print("Actual Required Margin:\t" + (actualRequiredMarginBefore - actualRequiredMarginAfter).ToString("0.00"));

            Stop();
        }
    }
}

I have run this on two pairs and the results are as follows:

First experiment:

Backtesting started
Account Currency Matches Quote
Account Currency: GBP
Symbol:                  EURGBP
Estimated Required Margin: 302.02
Actual Required Margin:    304.09
Backtesting was stopped

Second experiment:

Backtesting started
Account Currency Matches Base
Account Currency: GBP
Symbol:                 GBPUSD
Estimated Required Margin: 434.24
Actual Required Margin:    440.33
Backtesting was stopped
 

As you can see, the estimated are different from the actual. I tried using the Ask rather than the Bid, but that made a minor difference.

What am I missing to have an accurate estimate?