Some of the reason our figures differ see this pic below:
The precise instant the "red interior light" comes on may occur when only half the sun is hidden by the horizon. If so, it is not the correct, real life, sunrise/sunset.
We're dealing with a simulation and as such we must attempt to find the right combo that allows us to have some simularity of navigation. I personally would prefer greater accuracy than 60nm (1°).
Another navigational fault with the sim is the chart, as once again in real life you cannot draw a straight line course over long distances between two points. In RL a long straight line course is only drawn on a gnomonic projection chart to compensate with the curvature of the earth. A gnomonic projection is transferred to a Mercatur projection as a series of short rhumblines that resemble a curved course overall. Be that as it may, a sim cannot be all things to all players...so what I'm saying is we have the choice of getting lost (frequently) or dragging the ingame course icon from point A to point B.