When do i start putting citizens in my specialist slots? Im playing BNW and i dont know when to use my specialist slots. I feel like if i use them im denying food to my cities. Im playing on immortal and i notice if i use my specialist then my cities will have a much lower pop than the other civs cities(more than usual). Also for wide vs tall how do i distribute my population into specialist?
I almost never play a wide empire, so I'm speaking from the perspective of someone who plays tall 95% of the time.
Because you're asking this question, I'm assuming that you are going for a cultural or science victory. If you aren't going for those victory types, specialist slots aren't as important and you should fill them when/if you have the extra citizens. I mean, for domination/conquest victories, having specialists in science slots is a good idea and you should definitely do it; but, you probably won't have much time to focus on growing your cities so it becomes a little more difficult with those victory types.
Generally, you want to make sure that your city is working every tile worth working before you start aggressively pursuing slot specialists. Split focus between food and production early on, with maybe a little more attention being paid to growth. Switch to a production focus only when you need to.
It is important to plan your cities out before hand. Get an image in your head of the 3-tile radius a city can work and plan ahead. Know what tiles you simply must work to maximize the cities intended focus.
For a tradition player that runs tall empires, the focus in on cities with high populations. This means you'll be settling cities in areas with a high food potential. Again, early on the focus should be split between food and production, so all your citizens will be working tiles. Somewhere around the Renaissance Era is when you should be able to start funneling citizens into specialist slots while still getting a decent amount of growth.
Plan your techs accordingly so that you are getting boosts to growth when you need the extra citizens. These boosts come from buildings like Granaries, Aqueducts and Hospitals and technologies like Civil Service (Civil Service is an incredibly important tech for anyone looking to put specialists to good use) and Fertilizer. From the Renaissance Era on you should be able to add specialists at a reasonable pace for the rest of the game. By the Modern Era, your cities should be at their full potential... for some people this even happens in the Atomic Era. It just depends.
Food focus is absolutely essential if you want to use a good number of specialists. City-states and trade routes can help as well. Never forget those.