I mean, I think you need to look at the fact a forum of refs aren't answering that question and infer from there! The line is where you set it. End of.
There is a window in which you'll be happy with that line being set and each team (arguably, each player on each team, plus each team official, plus each spectator) will also have their own windows where they think you should be drawing that line. Interestingly, their windows might be different depending on if their player is the tackler or the tackle-ee. The window will also change depending on how they think you've been calling the game, the state of the pitch and a million other impossible to predict factors.
What you're imagining is a world where your window and their window overlap to an extent, and you then manage to draw the line inside that overlap and everyone is happy. It's not impossible, but a) even if the overlap does exist and you successfully identify it, you're human and still might struggle to hit the line consistently and b) the overlap simply might not exist at all. Which is why my preference is generally to set a line that I'm comfortable with and then just try to make my decisions around it as consistently as possible.
It might be because this is a bit of a pet hate, but I don't like players using a tackle as an excuse to try and clatter an opponent, so I'd have no issue carding for one of these if I thought the follow-through contact was remotely deliberate. And by the time you've given two of these to each team and explained what was unacceptable to the captains, anyone who then goes on to do this again after ignoring all the warning signs would have my hand twitching towards my pocket.
As long as you're setting a line consistently and explaining where that line is (either by talking or simply via your decisions), it's on the players to modify their actions to your standards, not the other way round. Same if the pitch is slippy and wet - that doesn't mean you raise the bar to allow players to just clatter each other, it means they need to modify their tackles to account for that.