Using a similar method to Nerdicus, I built 4 meadow farms with different fertility levels and a clay pit to act as a calendar (a clay pit completes 4% of its production every "day" (one 16th of a month)). I recorded the level of farm production every "day" for several months and then tried to come up with a simple and logical method that would explain the results. The method I came up with leaves a little to be desired in the "simple and logical" departments, but I have successfully used it to predict results for 4 other farms (some meadow and some floodplain, different crops), so I think it may be correct. This is how I think it works:
The basic farm production rate in units per month is 100 multiplied by the fertility level (so the basic rate for an 84% fertile farm is 84 units/month). This is converted into a percentage per "day" by dividing by 16 ("days" in a month) and by 800 (the number of units a farm produces if it completes its production cycle) and rounding to the nearest 0.05%. Each day, this percentage is added to the production level, but when you right click on the farm, the percentage you see is rounded down to the next lower whole number. At harvest time, the number of units harvested is calculated by multiplying 800 by the DISPLAYED percentage complete (the one rounded down to a whole number). Here is an example of an 84% fertile farm that operates for three days before being harvested. Each day's production is 84/(16*800) = 0.65625% which is rounded to 0.65%.
Day 1: production is 0.65% which displays as 0%
Day 2: production is 1.30% which displays as 1%
Day 3: production is 1.95% which displays as 1%
After day 3, 800*1% = 8 units are harvested.
Using this method, you can calculate the production rates for any farm, here are some examples (ignoring Osiris' blessing):
Ferility _ Units/mth _ Time to produce 800 units
25% ________ 25.6 ______ 31 months 04 days
50% ________ 51.2 ______ 15 months 10 days
60% ________ 57.6 ______ 13 months 15 days
70% ________ 70.4 ______ 11 months 06 days
80% ________ 83.2 ______ 09 months 10 days
90% ________ 89.6 ______ 08 months 15 days
100% ______ 102.4 ______ 07 months 13 days
So floodplain farms should be able to max out production with about 80% fertility if the labourer reaches it as soon as the flood receeds.
Meadow farms that have 1 harvest a year will max out production if the fertility is 68% or more. They are Chickpea (Apr), Lettuce (Apr), Fig (Sep) and Flax(Dec).
Meadow farms that have 2 harvests a year are more complicated. With 100% fertility, they can produce as follows:
Grain_: 800 in Jan, 408 in May (plus 100 straw each harvest)
Barley: 608 in Feb, 608 in Aug
Poms__: 712 in Jun, 512 in Nov
I'm assuming that the harvest date for meadow farms is the same for a particular crop regardless of what city you are in (unlike the floodplain farms) but I've only checked that on a couple of maps so I may be wrong. Sorry I've been so longwinded. If you're still awake, I invite your comments or corrections.