Keep track as you go through a gallon. All horizontal decking should be working out to about the normal coverage. If you're using more, there's no harm done, but you may be overspending. If less, then you may need just a little more to get it right. Redwood is a special case.
Most woody growth occurs in unconnected parallel lines up the trunk.
In redwood, however, these channels are all crosslinked and interconnected. As a result, a glass of water spilled on a redwood board will go throughout the wood if allowed to sit long enough. The trick with redwood is to wait a day or two between coats, so that early coats have a chance to set and help to inhibit absorption of later coats. It's possible to get an excellent finish on redwood with as few a three coats, though 4 or more are typical for an effective first application. Mill glaze will disappear naturally in 6-24 months, during which the wood will silver and weather damage will accrue.