How much should an specialist produce?

[Odd]

I always distribute the mines/irrigation so my cities only grow big enough to pick all tiles, because I don´t like the idea of trading 2 production points (or more, in many cases, if you have factories, power, or manuf.) for 1 point of science or gold. But I miss civ2 35-40 cities, so how much an specialist should produce to counter the loss of production because you irrigated instead of building a mine? 2? 3?