My SG1 was within a couple of percent of the mark at fill-up time, right out of the box. I just recently started using the correction factor to get it dialed in as close as possible.
They're also useful for showing you all kinds of info you can't get through your gauge cluster.
Yes. But to be clear, it's not a radiator block you're after, but a way to prevent excess airflow from entering the engine compartment in the first place. So you'd want to block the rear of the grille.