S.R.
Okay... If I'm understanding the question right... You're wanting to be done pumping by the end of the year. You're wanting to know when to cut back to pumping 6 oz per day.
19jars * 16 oz = 304 oz.
2 1/2 more months until he's a year old is about 30+30+15 = 75 days
x = # of days pumping 12 oz
y = # of days pumping 6 oz
x + y = 75
304 - (15-12)x - (15-6)y = 304 - 3x - 9y = 0 oz left at the end.
It's a system, but you can solve for x in the first equation giving you x = 75 - y. (The number of days you pump 12 oz is the total number of days minus the number you pump 6 oz.)
Substitute into the other equation to get:
304 - 3(75-y) - 9y = 0
304 - 225 +3y - 9y = 0
79 + 3y - 9y = 0
79 -6y = 0
79 = 6y
y = 13 1/6 days
x = 75 - 13 = 62
so pump 12 oz for about 62 days, then 6 oz for about 13 days.
If my 75 days isn't 100% accurate, then just follow the same steps I did, but change the 75 to however many days you need it to be. Everything else will be the same. Feel free to PM me if you have any specific questions or just need me to work it out with a specific number of days for you.