Looking at old original powder horns. You will see both lightly engraved and deep scratched heavily engraved examples. I don't think that there is really a set in granite rule. I personally engrave my horns deep. Why? Generally with my scriber(s), the deeper you cut the wider the cut and the engraving shows up better when inked or stained! I DO lightly engrave when drawing the detail of the horn, then go over everything again, deepening as I go! If you look at avatar on the left or some of my horn that are engraved, in the Premium Members Gallery. You can see how the engraving stands out in contrast to the horn body color! I never use #0000 steel wool to remove excess ink, as it takes off detail from your engraving. I think this is why we see very good engraving, but the engraver rubbed the detail away with steel wool, thus lightening the contrast. I do burnish (rub) the engraving with a glass bottle to knock off and smoothen any engraving chips or ridges.
You have to remember, that most of the old original powder horns that were engraved, were engraved by everyday folks and not just Horners and Gun builders! Engraving was done with whatever the person had available at the moment. Usually some type of knife or whatever "MADE HIS MARK"!
Rick Froehlich