I am currently experimenting with this formula:

197,000 lbs (operational empty weight) +
41,000 lbs (full load from ACE) +
  5,000 lbs (minimum landing fuel) +
FSX calculated fuel in nav log for trip +
35% (or 40% with headwinds) of FSX calculated fuel = Fuel Load

You can also try the FSX recommendation + 30,000 lbs as a rough and ready estimate.

The important thing is FSX will NOT give you a sufficient amount.  The question then become show much more do you need to make the trip safely and legally?

The problem with more "realistic" planning taking into account winds aloft and weights, etc.  is that I am not convinced that the effort pays off given the vagaries of FSX weather, even with add-on weather programs.  So try what you like, but I'll continue to experiment to find the easiest method.

The following shows you how to plan a realistic(ish) flight from Bahrain to London (OBBI-EGLL).

1. Find your real flight online.  You can use any one of number of flight information sources such as amadeus  (http://www.amadeus.net/) or the carrier's website.  For this one I used http://www.gulfair.com/bh/index.asp and found Gulf Air 005, departing at 1545 and arriving at 2100 for a total of 7 hours 15 minutes.

2. Load FSX.

3. Go to flight planner. Set up your departure and arrival airports and let FSX pick out your High Altitude IFR route.  Most of the time it will give you something pretty decent, but make sure.  Be sure to reset your altitude to something legal and reasonable.  (If you fly Aeroflot or Delta transatlantic routes in or out of Moscow, it may either refuse to work or route you in the wrong direction, so pick out your own routes on the map, point by point, along airways to the NAT.  It isn't as bad as it sounds once you get the hang of it, and you can save it as both an FSX flight and a company route.)

4.  If you want to use a Standard Instrument Departure (SID) and Standard Terminal Arrival Route (STAR) you will want to pay attention to where your SID ends and your STAR begins.  You can find charts online for nearly any airport and compare them with your route.  Your FMC has many such procedures.  Do not be surprised if your FMC and what you find online, or even in the FSX navigation database don't match 100%.  Depending on the date of creation, these things change (and FSX is frozen in time).  I'll have more about SIDs and STARs later.

For now, if you want to try them, just know that you will want the first fix on your route to be relatively close and in the same general direction of flight as the last fix of your SID.  If you can use the actual transition, great.  That is what is intended for: transitioning you from your procedure to your route.  The same goes for your STAR.  You will want the last fix of your route to feed naturally into your STAR.  For example, if you are coming into Moscow from the west, you will not want a STAR that starts way over somewhere to the east.

If your navigation database used by the FMC has procedures for an airport, you can find them on the Departure and Arrival page.  Pick the appropriate runway and transitions.  (Again this is just general.  I'll have something in detail later.)