According to USDA tomatoes in canning require a pH of 4.6 and lower. Correct me if I am wrong.
Here a list of some varieties with pH of LOWER than 4.6. This means that they can be safely canned without further acidification. To be on the safe side, less than 1/2 TBSpoon of lime/lemon juice or vinegar per quart, can lower the pH much further, likely to under 4, well into/within safe range.
NOTE of acknowledgement:
The list is provided by The University Of Illinois , according to Pupila C in tomato forum. The first number after the name represents avrg. tomato size, in oz and the second number is the pH.

So when we talk about tomato acidification in canning, this information can give an idea what we are talking about.

Federle 6.5 4.59
Pete's Italian Plum 4.7 4.56
Zapotec Pink Pleated 6.3 4.56
Jersey Devil 5.2 4.55
New Jersey 5.3 4.55
Spitze Russia 7.3 4.54
Large Polish Paste 8.2 4.53
Bisignano #2 7.3 4.52
Opalka 5.9 4.51

UNDER 4.50
Rutgers 6.8 4.51
Burbank Red Slicing 4.2 4.49
Tondo Fario 5.7 4.49
Black Krim 9.4 4.48
Grandma Mary's Paste 3.4 4.47
Marglobe 5.8 4.47
Rio Grande 3.0 4.47
Super Sioux 5.0 4.47
Le Case di Puligia 1.0 4.46
Soldacky 13.5 4.46
Ten Fingers of Naples 1.9 4.46
Brandywine, Pink 14.1 4.45
Rio de Fuego 2.5 4.45
Hungarian Heart 14.1 4.44
Kavalarovo Plum 1.1 4.44
Antique Roman 6.9 4.43
Homestead 8.9 4.43
Amana Orange 15.4 4.42
Amish Paste 7.4 4.41
Joe's Plum 10.2 4.41
Juane Peach 1.6 4.41
Goliath 16.1 4.40
Howard German 4.1 4.40
Polish Linguisa 6.0 4.40

UNDER 4.40

Rose de Berne 5.4 4.40
Delicious 6.2 4.39
Italian Plum Canning Type 3.0 4.39
Moskvich 6.2 4.39
Brandywine, OTV 10.7 4.38
Cherokee Purple 8.6 4.38
Martino Roma 1.9 4.38
Napoli 1.7 4.38
Popovich 3.1 4.38
Santiam 3.2 4.38
Arkansas Traveler 4.8 4.37
Brandywine, Yellow 13.2 4.37
Di Santos 7.7 4.37
Costoluto Genovese 4.9 4.36
Saucy 1.8 4.36
Anna Banana 5.9 4.35
Black From Tula 9.9 4.35
Ellis Island 14.7 4.35
Stupice 2.2 4.35
Ernie's Plump 6.9 4.34
San Marzano 2.1 4.34
Watermelon 14.2 4.34
Super Italian Paste 5.4 4.33
Bloody Butcher 2.6 4.31
Jubilee 6.9 4.31
Striped Stuffer 3.9 4.31

UNDER 4.30

Giant Syrian 10.6 4.29
Landino di Panocchia 3.5 4.29
Laurano 2.5 4.28
Mortgage Lifter 12.8 4.28
Pantano Romanesco 7.8 4.28
Caspian Pink 12.4 4.27
Persimmons 9.0 4.27
Bull's Heart 9.3 4.26
Ropreco Plum 1.8 4.24
New Hampshire Surecrop 1.0 4.23
Pomodoro Grosso 5.4 4.22
Oscar 6.3 4.21
Silvery Fir Tree 3.8 4.21
Abe Lincoln 4.9 4.20

UNDER 4.2O

Bianco Great White 9.2 4.16
Prairie Fire 2.3 4.10
Purple Calabash 4.0 4.07
Platillo 4.1 4.02