Thanks, I didn't measure the metal thickness exactly but it's around 2.5mm.
I don't think my first bit had massive penetration, then I wound up the amps and nearly started blowing holes! backed off a touch and that seemed good.
Not bad at all. Looks like you have the settings very near. Your not really going to get any full pen on a crack, unless you keyhole it, and as you said, you were blowing holes. Practice, practice, practice. As they say.