Downwind,
when it comes to flight planning calculations such as this it is perhaps best to consider the equations in the context of the problem.
What do you want to do?
You want to know whether or not [under various conditions of wind and airplane configuration OEI and OEI/depressurized] how long can I fly before I cannot possibly make it back to my origin? [at least fuel wise]
The specific answer to that question would be a function of how fast you're going over the ground---however you determine that.
how much fuel are you carrying and burning i.e endurance---however you determine that.
Now, take a look at the equation above and see that if you peel away the layers [GS from wind component {plus all of those tedious compass corrections} TAS from EAS and all the AFM/POH data
etc.] as required. However, in spite of these complications, the relationship that evolves is actually a very simple one the simplest in fact as alluded above, and an E6B [there's no teaching or learning value in electronic flight computers, what do they ever elucidate about altimetry?!!] and a few hours of play makes these calculations a little fun
My response is NOT meant to seem sarcastic or condescending I've seen the Aussie ATPL material!!!, but sometimes because of the nature of aviation and pilot training which is supposed to be soooo difficult
...
We can lose the simplicity and freedom of aviation but with time and some cognitive reprogramming we regain a clear view again. But, remember you're totally professional from the moment you soloed in command of a Cub as this is never really a hobby, now that the trauma of your examination(s)! is over... You may again rejoin your mind with the simplicity of [Airplane] flight and now enjoy the true to life esoterica of aviation like the kinematic viscosity of the atmosphere and NACA airfoils, Good Luck!!!