There’s are a few test templates out there that can really speed things up.
As for how dark: generally things get darker the slower and higher LPI you go. You can get a pretty dark engrave without much depth by doing relatively low power and slow speed with higher lpi.
Granted, that will take longer, but there you go.
This thread will be relevant for you. Ignore that it’s about HTV, just look at the discussion about testing.