Once we have reached a surplus of 12 hammers/shields, it will take us 10 turns per worker to produce one. We can consider this 250 turns to produce half of the 50 in one city (since the cities will produce one or two of their own). If we stagger this with 25 turns to build a settler, this will result in a worker being produced in the city every 35 turns. After 350 turns, this will be 10 workers, and might be enough for this city, on account of each new city being assigned to build 1 or 2 workers themselves.
I don't think of worker or settler builds in terms of surplus hammers. As soon as slavery is available and city regrowth is possible I try and whip these units - it's more efficient.
In terms of new cities building workers, I avoid this beyond my first 1-2 cities as I'd rather they started growing and contributing ASAP. Try to have a worker go to improve a new city at the same time as settling it or the new city is just a drain on your empire.