<Ivor>
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The issue isn't just about misclicks; even if you remember not to click a second time, you still have to click somewhere else to calculate the intended route, which wastes time.
I think it would be optimal to assign this function to the right mouse button, even without using Alt, so that having a beer in your left hand wouldn't interfere.
Time will actually be wasted on the extra right-click. In any case, you need to click with the left mouse button first to plan the route.
Here's an option with an additional display when right-clicking, alongside the existing Alt+left-click: http://zalil.ru/32977522
Unzip the file and place it in the Data\c folder.
The right-click display will only work if the last left-click was in that same location.