That equation is basically correct. Point is to identify the speed (and altitude) to minimize fuel burn. You could simply decide to measure it yourself deciding a cruise altitude and speed and then sampling your fuel burn rate (checking how much fuel you spend in a minute). You can do that with half fuel load to have an average cost (it probably changes if you are fully loaded or with empty tanks)
Plus I suggest you to check how much fuel you spend to gain cruise altitude and speed and subtract that amount from the fuel you use to calculate range available. Taking off and reaching cruise altitude can be an important part of your fuel cost (expecially in short range flight)
Just in case someone would be in the same need the answer is
target:geoposition:heading
it gives you the (local body) heading (north based) from the ship to the target.
Joe Kerbal's post in How do I Calculate Aircraft Range was marked as the answer