I have never had a wine drop .5 in ta. As a rule of thumb, I like to maintain a ta of .65 (for reds) and 70 (for whites). I do 3 adjustments.
First adjustment - prior to pitching yeast
Prior to pitching yeast, I like to see a TA of at least .55. Acid is important in color extraction during maceration, so I will adjust the TA if any lower.
I choose .55 at this point because any reading I make is inaccurate (sample has pulp and other suspended matter in it). Additionally, any adjustment I make is just a guess as I do not know exactly how much wine I am dealing with.
Second adjustment - at second racking (at week 1 or 2)
At second racking, I measure and readjust the wine, only this time up to .60 (for reds) and .70 (for whites). This gets me close to my ultimate goal. I feel ok to get this close to my ultimate because I can now draw a much more pure sample and also know the exact volume I am dealing with. I will/may lower or raise this adjustment based on the wine's PH.
Third adjustment - at third racking (at month 2 or 3)
Final adjustment (if needed). Any adjustment here is mandated by taste.