There seems to be confusion between 'is it worth working food-deficit hammer tiles at all' and 'is it better than working an additional grassland farm'.
With a Granary, the conversion rate is roughly 30

: (10+size)
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First the trickier bit, to answer the actual question of
'is it better than another grassland farm?'
For now, I'm assuming we are unconstrained by the happy cap and considering a 1-point whip - pretty much as good as it gets for Slavery.
I'm also assuming that we have a food equilibrium when working the hammer tile and any food saved by not supporting that tile would go towards whipping.
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A grassland Forest feeds itself and requires 30 turns to produce enough hammers to be equivalent to a 1-pop-whip (which I'll use as a standard to simplify things).
A grassland farm gives you a net 30

in that time, which translates into more production via the whip below size 20.
Plains Forest: You need to work it 15 turns for 30

Again, working a Grassland Farm instead gives you 30 more

in that time period and is superior below size 20.
Note that this doesn't imply all forests are equal - the period length is shorter for the plains forest! In short, below size 20 1

whipped > 1

.
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Grassland Hill: At 1

deficit, 10 rounds are needed to produce the 30

equivalent to a 1-pop whip. A Grassland Farm would result in an additional 20

.
Below size 10, a farm for whipping will do more; this is of particular interest to people torn between Slavery and Caste System for workshops equivalent to a grassland hill.
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Plains Hill: 7.5 rounds needed for 30

, at a total food deficit of 15

. In that time period, a grassland farm would result in 22.5

to play with.
At size 12 the grassland farm produces better results, at size 13 and above the plains hill is superior.
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*****
Some thing many people confuse with this for some reason:
'Is it worth working a food-deficit production tile at all, even if we have no more food-surplus tiles available?'. This one is easy.
Grassland Forests give a hammer at no investment of food, so they're worth growing into, even if they are thoroughly unexciting.
Plains Forests and Mined Plains Hills both convert food into hammers at a rate of 2

: 1

. Below size 6 we shouldn't bother and simply whip.
Grassland Hills give a conversion of 3

: 1

, which is better than anything attainable by the whip.