When do you count the main crop season starting for a particular variety? 

When the first one ripens of that variety?  What if one main crop develops abnormally early?  This year I have had at least 4 varieties push out a ripe fig much earlier than the rest of the crop – Black Bethlehem, Bryant Dark, Conadria and Nero 600m.  In each case the single early ripe fig was very large, it was NOT the lowest on that branch (usually they ripen from bottom to top or oldest to youngest) and in each case the rest of the figs are not near ripe.  Generally, in my limited experience, the second ripe fig on a plant starts developing within a few days of the first.  Maybe it is because these are relatively young plants (Bryant Dark 3^rd year, others 2^nd)?

 So when would you count the date of first ripening – when the first one occurs if it seems to be an outlier?  Or when the rest start ripening?  Especially for the Nero 600m, this was weeks ahead of when it should be ripening and it was NOT a breba.

Ed, I had an Atreano cutting rooted this spring that ripened  a fig Aug 13.  The main Atreano plant is just now swelling its main crop.  I don't think an odd fig ripening early establishes the ripening date. 
IMO it should be the date when a substantial portion of the crop ripens and averaged over several seasons.
it will be interesting to see how others establish the ripening date of their fig cultivars.

Ripening dates are affected by many things. I would consider the ripening date to be the date when there is a significant portion of the crop somewhere in the ripening process. If you have 6 figs on each branch, that would mean that 17% of the crop could ripen on the first day, and they rest would follow in a progression, so a significant portion would be 17%, assuming that the first fig on each branch formed at the same time. But there are other factors that could change the way thing progress. the North side of the tree could get less sun/heat and be delayed in ripening. A portion of the tree might be in the shade. One side might get reflected heat from a driveway or wall. So maybe a significant % is more like 5-10%

In California's Central Valley, their picking window is only 2, maybe three weeks, from the first ripe fig to the last, so their significant % is 15-15%, and they have no problem figuring out what their ripening date is - not by the calendar, though that is a good starting place, but by observing the fruit.