I generally look at the city terrain, count out the resulting food balance when I'm working all decent tiles and have made "productive" improvements to them, and then build just enough farms to swing the theoretical balance to zero. I'll make an exception for 1-2 squares not earmarked for eventual townhood if that's necessary to get me a decent early growth rate, but that's about it.
I'm not really sold on the all-farm GP factory concept. I find that my first Great Scientist is disproportionately valuable because of its synergy with Bureaucracy, but beyond that, I'd rather get my GPs at the "natural" rate that National Epic + bunch of other Wonders in same city + 1-2 specialists delivers, and try to get all the GPs that trigger from discovering a tech first. I still get a reasonable number of GPs this way, without raising their cost too much.