Calculating additions at crush have always been a bit problematic for me.
Never seem to come out right. So usually put half calc'd amount of Tartaric
acid in for example and then check affect the next day and adjust again.
Based on actual impact. I have a theory this year that maybe all that is
immediately affected by this is amount of free run juice that's in the
fermenter after crush. Let me explain by looking a batch of Cabernet
Sauvignon wine I'm making this year.

Grapes:

325 lbs of Cabernet Sauvignon + 38 lbs of Merlot

Initial readings:

Brix 22.5
pH 3.53
TA 0.52

(Yep, over mature grapes that didn't reach 24 brix)

Based on past experience I calculate:

31.9 gallons of must
26.6 gallons of wine after press made up of 17.8 gal of free run and 8.8
gal of pressed juice

Ok on to initial calculation

Goal: Bring sugar level up to 23.5 Brix

1.5 oz of sugar will raise Brix in 1 gal wine 1 brix

Sugar addition: 1.5 oz sugar /gal x (23.5 Brix (desired) -22.5 Brix
(initial)) x amount of wine after pressing (26.6 gal) = 39.9 oz of sugar
(round to 40 oz)

Ok added that.. next day.. Brix 24.2 What the heck!! Ok lets back out
how many gallons should have been used in calc then..

40 ozs sugar/(1.5 ozs (sugar/gal) x (24.2 Brix (actual) - 22.5 Brix
(initial)) = 15.6 gal

So if I had used 15.6 gal in my original formula the world would have been a
perfect place. Ok lets go on to Tartaric acid addition

I'd like to get pH down to 3.35 +/- and total acid up to 0.7 or 0.75. Plan
on putting wine through Malolactic fermentation and that hopefully will keep
acid and pH in desired finished wine range.. but I'm only going to add about
half and see what happens first. I'll shoot for TA of 0.63 for starters
then.. Here's formula..

Goal: Bring TA up to 0.63

3.8 gms tartaric acid will raise TA of 1 gal wine approx 0.1:

Tartaric acid addition: 3.8 gms tartaric acid/gal x (0.63 (desired TA) -
0.52 (initial TA)) x 10 x amount of wine after pressing (26.6 gal) = 111.3
gms of Tartaric acid to add (round to 111.5 gms)

Ok added that.. next day TA 0.73, pH 3.33.. Yikes!! that's a close call..

Lets back out how many gallons should have been used in calc then to get TA
of 0.73..!

111.5 gm tartaric/(3.8 gms (tartaric/gal) x ( 0.73 (actual TA) - 0.53
(initial TA) x 10) = 14.6 gal

Ok so based on this gallons used in these two calculations should have been
14.6 to 15.6 gal and that's all the wine that was affected by the additions.
What gives here?

So here's my head scratching theory... Based on past experience I'm
predicting with prettty good accuracy free run at time of press to be 17.8
gallons. Initially after crush, amount of free run juice is probably less
than that and yeast will release more juice.. This is gettng close to the
14.6 to 15.6 gal range in backed out calculations above. Soooo, I'm
starting to think maybe these additions are only affecting the free run
juice in the fermenter and readings the next day are reflecting that. What
do you think..? Does this make sense? Are we only adjusting the free run
juice initially?