The only comment I can make is "do you have experience with automotive electrical systems?"

Newer cars might be different, but the voltage used to vary a lot. I'd just make sure to have a circuit that can withstand up to 25V without breaking a sweat.

Oh yeah, I read a while back about some manufacturers wanting to use 24V and up, just a thought.

As for your circuit VS Charles divider or any other design, I'm not qualified to comment.

Robert