I've never been clear on what volumes to test/adjust with respect to
both pH and TA in red must. Having just picked up a 1000lbs of Merlot,
I'm really anxious to get the acid adjustment right before my primary
fermentation ends.

At crush, I took a sample of must, both free-run and grapes, attempted
to press and strain the combined juice, then titrated TA and measured
pH. My results showed:

TA of 4.25 g/L
pH of 3.82
23.25 Brix

I decided to initially raise the acid by 1 gram of tartaric. My first
question is how should that 1 gram per litre be calculated, on the
volume of the must (420L) or the volume of expected juice after
pressing (approx. 310L - according to literature I've read). I chose
the latter and added 310g of tartaric to my must.

I tested the following day using my 'press and strain' technique
and got the following:

TA 5.75
pH 3.63
23.50 Brix

The sample also tasted pretty bland.

I also drew a sample of free-run and tested the pH, which was 3.39.
This sample tasted very tart.

Now, I'm torn on whether or not to add more acid (I started the
ferment last night). I understand starting a ferment above pH 3.5 is a
bad idea, but which number (pressed vs. free-run) has more meaning?
Pressing seems really inaccurate, simply because one can't ensure a
consistent ratio of free-run to berries and the berries don't press
very well. I'm also concerned that dropping the pH of the free-run
any lower will cause problems for my ML, which I plan to inoculate
towards the end of ferment.

Any help would be most graciously appreciated.

Darin