Ahhh But you have to committ to a mission. That feature does not start to happen until you select sail or mine or bombard ... Now select mine .. decide to change your mind after selecting and in my limited experince the only options are patrol, sail, return to base and the mines all go away ... unless I am doing something wrong ..
You don't have to commit to anything. Select a unit and drag the green arrow around the map. While you're doing this, the white text at the bottom of the screen will display the time required by the selected unit to make a round trip along the green arrow to the position of the cursor, and then from there straight back home (it also displays the total distance of the round trip, if you're worried about having the necessary range).
Thus, if you've done this carefully (see below), you can divide the round trip time in half to determine when to sail the unit to have it arrive at the objective at the desired time. In fact, you can use this to coordinate several units from different bases with different cruising speeds so they all arrive in the same place at the same time. And because you're just drawing green arrows but not actually issuing orders, you can deselect units at any time and they'll still be in port with no orders.
There are 2 things to remember while doing this. First, it takes 1 hour for units to get underway after you give them movement orders, so after you divide the round trip time by 2, remember to add 1 more hour to this to get the actual time to give the saililng order.
Second, to be able to divide the round-trip time by 2, the distance traveled both to and from the objective has to be the same. This is only an issue if you use waypoints to specify an indirect ingress path. As said above, the the round-trip time calculator assumes a straight shot back to base from the current position of the cursor, so if you have an indirect ingress and leave the cursor on the objective, the ingress time will be longer than the egress time so you can't divide by 2. To work around this, use more waypoints to specify an egress path tracing the ingress path as close as possible, even if you really want to take the direct route back when you actually execute the mission. Now the calculator will have about the same distance on both sides of the objective, so you can divide the round-trip time by 2, which is what you want.
Remember, this is all just planning at this state, so you're not committed to leaving base at all, let alone coming home by the same route you went out. The only reason for retracing the route is so you can get an accurate idea of how long ingress on that route will take, which is the purpose of the whole exercise
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So for example, I want GF, BCF, and HF all to arrive near Horn Reef with H-Hour at 0500. All of them are going to take direct routes there so no worries about waypoints and I can just divide the displayed travel time by 2. Playing with the green arrows, I discover that the round trip time shown for Tyrwhitt is 20 hours, 22 for Beatty, and 27 for Jellicoe (more or less--this is just an example roughly based on memory). For them all to arrive at 0500, the time to give the order to each of them would be as follows:
HF: 20/2 +1 = H -11 hours = 1800 the day before.
BCF: 22/2 + 1 = H -12 hours = 1700 the day before.
GF: 27/2 +1 = H -14.5 hours = 1430 the day before.
I write all this down and watch the clock. As it approaches the time for each TF to sail, I slow the time acceleration, pause the game at the desired time, and give the order to the appropriate unit, repeat.
