Start with your flat-water pace. A following current is added. An opposing current is subtracted, and the speed does not drop below half a mile an hour. A headwind multiplies that speed by a factor that bottoms out at 0.55. A tailwind adds a smaller boost. A crosswind is a lighter drag.
The route is four legs. Breaks are spread across them. If sunset minus your buffer is sooner than the finish, an out-and-back names the latest turnaround that still gets you back. A point-to-point has no reverse. The deadline is simply off the water before that clock.
The total is free. $5 shows each leg and the turnaround card.