Alex, i'll try to explain it differently, see if this helps.
say you have a battery that's 1000ma for example. You can simplify that by saying it's a 1 amp battery. 1000ma=1amp, with me so far? So, knowing that, a 2500ma pack is a 2.5amp pack, and so on, ok? I know that seems a bit confusing, when you think that the motors draw higher amperage numbers than that, but that's where the discharge rate comes in.
For example, a1000ma, or 1 amp pack, at 10C, can deliver 10amps of current safely. Most packs also give you a burst rating. Say that same 1000ma 10C pack has a burst rating of 15C for 15 seconds. That pack can safely deliver a burst of 15 amps for 15 seconds without damage to the pack. With me so far?
So what that means, say you have a plane with a motor and prop setup that will draw 10 amps at full throttle, and you need a battery pack that will safely run that setup without excess weight-remember, the larger the pack, the more it weighs-. If you decided to run that 1000ma 10C pack, it would be ok for that plane. You wouldn't want to run a 5000ma pack on that plane, because it wouldn't even leave the ground with that much weight. Understand? The opposite is true also, you wouldn't want to try to run a 500ma pack, you wouldfry the battery by discharging it too quickly by exceeding it's discharge rate.
So, knowing that info, your requirement of .4 amps is well within the capacity of the battery you want to run, the battery is way overkill actually. Be sure that the voltage rating for your battery isn't too high for the camera transmitteryou're going to run, or you'll probably fry it. You could run a much smaller pack and still be ok.
If you had a 2500ma 3s 20C pack, that pack would be capable of delivering 50amps at 11.1 volt continuous. 3s @ 3.7v/s= 11.1v. -2500ma, or 2.5a x 20C = 50 amps.
You get it now? That is, of course, if the battery manufacturer is honest with their advertised capacity and rates. It's best to go a little higher capacity than you need, just so you're not pushing all of your equipment to the max all the time. It's the same as when you get an ESC, you always want some reserve capacity, but not too much to where you add unwanted weight.