To estimate harvest date: Graph your Brix versus TA and Brix versus pH based on days.
That is:
In a graph, put day you sample on the horizontal axis. On the left vertical axis, plot Brix. On the right vertical axis plot TA.
Then in another graph, also day sampled is the horizontal axis, plot Brix on the left and pH on the right (but plot pH declining --- that pH 2 at the top, pH 4 at the bottom).
As you get data you can start to see where the two lines (Brix and TA or Brix and pH) will cross by date. Harvest time is nearest the estimated point where these lines will cross.
Caveat: If TA or pH gets too out of line for a good wine, harvest before they do, no matter what graph says or what the Brix is.
Also check your Brix * pH^2 (Brix times pH squared). Harvest when it is 200 for whites, 260 for reds.
Hope this helps.