Hello
The simplest way I can think of to let cows be retired from herd early and reflect the change to the estimation is to build a Herd table which records date joined and date left for each cow and calculate the number of cows of the i'th month in herd at any given time based upon the Herd table.
Here's an example tables built in Numbers v2.
Estimation (excerpt)
A1 month
A2 2015-01-01
A3 2015-02-01
B1 reference date
B2 =EOMONTH(A2,-1)+15
B3 =EOMONTH(A3,-1)+15
C1 cows 1 mo
C2 =COUNTIFS(Herd::$B,"<="&EDATE($B2,3-COLUMN()),Herd::$B,">"&EDATE($B2,2-COLUMN()),Herd::$C,">"&$B2)
C3 =COUNTIFS(Herd::$B,"<="&EDATE($B3,3-COLUMN()),Herd::$B,">"&EDATE($B3,2-COLUMN()),Herd::$C,">"&$B3)
D1 cows 2 mo
D2 =COUNTIFS(Herd::$B,"<="&EDATE($B2,3-COLUMN()),Herd::$B,">"&EDATE($B2,2-COLUMN()),Herd::$C,">"&$B2)
D3 =COUNTIFS(Herd::$B,"<="&EDATE($B3,3-COLUMN()),Herd::$B,">"&EDATE($B3,2-COLUMN()),Herd::$C,">"&$B3)
E1 cows 3 mo
E2 =COUNTIFS(Herd::$B,"<="&EDATE($B2,3-COLUMN()),Herd::$B,">"&EDATE($B2,2-COLUMN()),Herd::$C,">"&$B2)
E3 =COUNTIFS(Herd::$B,"<="&EDATE($B3,3-COLUMN()),Herd::$B,">"&EDATE($B3,2-COLUMN()),Herd::$C,">"&$B3)
F1 cows 4 mo
F2 =COUNTIFS(Herd::$B,"<="&EDATE($B2,3-COLUMN()),Herd::$B,">"&EDATE($B2,2-COLUMN()),Herd::$C,">"&$B2)
F3 =COUNTIFS(Herd::$B,"<="&EDATE($B3,3-COLUMN()),Herd::$B,">"&EDATE($B3,2-COLUMN()),Herd::$C,">"&$B3)
G1 cows 5 mo
G2 =COUNTIFS(Herd::$B,"<="&EDATE($B2,3-COLUMN()),Herd::$B,">"&EDATE($B2,2-COLUMN()),Herd::$C,">"&$B2)
G3 =COUNTIFS(Herd::$B,"<="&EDATE($B3,3-COLUMN()),Herd::$B,">"&EDATE($B3,2-COLUMN()),Herd::$C,">"&$B3)
H1 cows 6 mo
H2 =COUNTIFS(Herd::$B,"<="&EDATE($B2,3-COLUMN()),Herd::$B,">"&EDATE($B2,2-COLUMN()),Herd::$C,">"&$B2)
H3 =COUNTIFS(Herd::$B,"<="&EDATE($B3,3-COLUMN()),Herd::$B,">"&EDATE($B3,2-COLUMN()),Herd::$C,">"&$B3)
I1 herd size
I2 =SUM(C2:H2)
I3 =SUM(C3:H3)
J1 total litres
J2 =SUMPRODUCT(C2:H2,Litres::B$2:G$2)
J3 =SUMPRODUCT(C3:H3,Litres::B$2:G$2)
K1 litres/cow
K2 =J2/I2
K3 =J3/I3
Notes.
The formula in C2 can be filled down and right across C2:H13.
C2 is calculating the number of cows of the 1st month in herd at the time of reference date = 2015-01-15 by means of COUNTIFS() counting cows in Herd table where 2014-12-15 < [date joined] <= 2015-01-15 and [date left] > 2015-01-15.
D2 is calculating the number of cows of the 2nd month in herd at the time of reference date = 2015-01-15 by means of COUNTIFS() counting cows in Herd table where 2014-11-15 < [date joined] <= 2014-12-15 and [date left] > 2015-01-15.
And so on.
Good luck,
H
EDIT: replaced the screen shot with correct one.
