I'm sure you'll get the hang of etching, give it a go on a few scraps. For what it is worth, I barely touch the surface, moving the blade sideways, as if to scratch/scrape, and definitely not to cut. I work using the very tip of a surgical blade, but I've seen Alan Ross use an Xacto knife in exactly the same way. It takes a few "touches" before anything disappears, but I'd rather make it go lighter than disappear, as the traces will be less visible that way. Altogether, if the line is very geometric on a smooth, lighter tone, this technique is unlikely to work in my hand. If the background is very light, bleach seems easier, and if it is a busy spot, anything is good.

I've noticed Alan used two Xacto knives, one was new and sharp, the other he blunted purposefully, by grinding its top round, and he used them for different spots, but I fail to remember what the blunt one was for, I'll ask him.

And if you need perfection, there's Phyllis, in US, and there's nothing she couldn't spot.