Adding 2nd landing Light

That should be fine,

  You only have the landing light on for a few seconds a flight and if it is the bit that exceeds the alternator then the battery will provide the needed power.

Determining you total load is not easy, you can add up all your instruments etc. The easiest way is to on the ground put an ammeter on the battery and see the draw with everything on, this depends very much on what , if any instruments you have....

The ignition circuit is separate and dedicated so should not effect the calc ??

Glenn mentioned that landing lights would only be turned on for a few seconds a flight...  That may or may not be correct, depending on how the landing lights are used.

When used as "recognition lights" in addition to actually using as landing lights, that circuit would most likely be ON basically from just before the first takeoff until after the last landing.  If the amperage load with the additional light exceeds the alternator's capability, you'll eventually have a dead battery situation.

I would argue that since you don't know who might operate your aircraft at some point in the future, it is incumbent on you (the person making the mods) to perform the electrical analysis and determine if you're within a safe range for your electrical system.  

Vans should already have the total amp draw on record since they designed the aircraft.

The UL/ULS built-in alternator is rated at 250W, but it's basically an outdated system used on old Ducati's, like my 1998 ST2. Full output is ~18 Amps but that only happens at near full rpm and it's not advisable to pull that much continuously. In an older video, Lockwood strongly advises not tp pull more than ~12A continuously from this system.