That's a fairly big pot. How tall is it, in inches? If you intend on keeping it in a pot for it's life duration, that's probably big enough. Also depends on whether you want to keep it under a certain height, but that could be determined by other factors as much as pot size. But usually, the bigger the pot, the bigger the plant, so it's up to you.
The yellowing of the oldest leaf is indeed an indication of nitrogen deficiency, as you guessed. That leaf will probably die and drop off soon, but as long as the yellowing doesn't continue in other leaves, there's no problem.
When watering, the top of the medium will dry out a lot faster than the bottom, so you may be over doing it, but it's not showing any signs of overwatering in that pic, so you're probably ok to continue the same watering schedule. It can actually be good for the plant if you let the medium dry out, top to bottom, so long as the temperature isn't too hot. This encourages the roots to expand in search of water. But there should be holes in the bottom of the pot, and I usually judge when to water when it's dry around those holes.
All in all, you seem to be doing fine. It looks pretty healthy, if a little stretched. Just give it as much sun as possible. Even leave it outside 24/7 if it doesn't get too cold at night (as in frost cold), so it can catch every bit of sunlight possible.
