Just want a double check on something: the reference markings found on the front of most bars, they refer to the horizon for level, correct?

It other words, with the typical rise/sweep MTB bars, to get the max rise/sweep you want the ref marks to be level with the ground (NOT relative to the stem).

Maybe, but there are probably too many different manufacturers to generalize. If you want max rise just rotate the bars, watch the grip area and stop at its peak.