Please help a newbie

[BoSox]

And the same would be true -- that you can use the RC instead of TC -- for 16 v 10, which, like 12 v 4, is to be hit only in negative counts? Because any negative number divided by a positive number equals a negative number?

No, composition dependent basic strategy comes into play based on the number of decks used and whether the game is S17 or H17. So there are times when the true count is zero where you would hit a 12 vs a 4 up card. Such as in a S17 6 deck game, or less and your two card 12 has a ten value card in it requiring you to hit.