In Wales I've been a fair bit more strict with it, especially at the start most of us as referees potentially had games with very high penalty counts as a result. The main difficulty we've had has been judging the 2nd tackler as the ball carrier is usually on the way down - this is where most of the debate has come in.
The simple answer at the moment for me is be practical and empathetic in your judgement, if it looks wrong, it probably is, but equally be firm in it because otherwise behaviours won't change. Over time I'm seeing tackling getting better and better, and teams disciplining themselves when they see the tackle heights creeping up.
I agree with Phil it is useful to remind yourself that it is 'open play' only. If players attempt to tackle to prevent the offload or rip the ball, they are now running the risk of being penalised for it, I want to see a tackler get low, bent at the hips, and attempting to wrap to make a legal tackle - if they do that, then usually they've done their part in making a legal tackle, its then the ball carriers responsibility to not dip excessively and cause injury.
Doesn't mean coaches and spectators see it the same way of course, as per my last game...
Red player: *clearly tackles above the sternum*
Me: "Penalty advantage, high tackle"
Red Coach: "C'mon ref! That was never high, he tackled the chest!"
Me: ...
(Obvious note: this was AFTER my team talk at the start of the game with them all in the changing room talking about the new tackle height...)
