The flight planner is designed to work with a simpler navigation technique designed to reduce pilot load when flying. In most (all?) cases a flight will not go exactly as planned. The weather predictions are almost always incorrect, the planned cruising altitude and planned track may also be different due to weather.
The technique is to use nil-wind when creating the flight plan, which allows easy updating of values before and in flight. The basic concept is that the plan will change, so the generated flight-plan makes it easy to update time and fuel values based on this assumption.
Firstly you will need your E6B flight computer. This has a circular slide rule around the outside. The circular slide rule is just a ratio device. If you set the outside ring on 80 and the inside ring on 40 (written as 80:40), you can look around the wheel and see that all values are at a ratio of 2:1. For example, look at 30 on the outside ring and you see 15 on the inside ring, 30:15. This is all the circular slide rule does; ratios. It doesn't matter what ratio you set, the same ratio is used all around the circular slide rule. e.g. 15:90 is a ratio of 1:6 which can be read around the side as 4:24.
So remembering how the E6B works, we try to get all our calculations from the one ratio. That is, we want to set the E6B on one value and then try to read off all our calcuations from that. Traditional methods for calculating distance travelled to time taken means constant twirling of the E6B, so avoiding that will make navigation simpler and less error prone.
It turns out that there is a way to set the E6B to simplify use in the cockpit. Before takeoff we can set the planned speed against the expected speed to obtain the ratio we require. For example if we were planning to cruise at 100Kts and the latest weather report shows a 20Kt headwind, we know we will have a ground speed of 80Kts. So we can set 100Kts against 80Kts (100:80). If our expected time for the leg is 20mins, we can now look at 20 on the inside ring and see that it is opposite to 25 (25:20). Without adjusting the E6B we have now been able to read off our new time interval based on the expected ground speed.
Not only for time calculations, we can also calculate fuel used. If we expected that we would burn 10 liters of fuel on the first leg, we can look across from 10 and see that we will now burn 12.5 liters (12.5:10).
So now, without changing the E6B setting, updated times and fuel usage can easily be obtained.
The E6B can be updated in flight. If we expected to fly a leg in 40 mins and it actually takes 30 mins then we set the E6B to 30:40. We can then look across from our planned speed of 100Kts and read off our ground speed as 133Kts (100:133). Then we can look at our expected fuel usage for that leg, say 20 liters, and see that we should have used 15 liters (15:20).
So by working in ratios from planned nil-wind values it is easy to calculate speed, time and fuel used based on actual values for the trip. This is significantly easier than trying to work out speed from distance and time values, then working out fuel used from fuel rate and time values. Also, since the planned values are known to the pilot there is less chance of making a mistake. If you were to use different TAS for each leg, based on predicted (and ultimately incorrect) winds, you may have a harder time trying to update your navigation based on actual figures.
Based on this simpler approach to flight planning and navigation, the flight plan generated by the flight planning program calculates values to-go. e.g. ETI to-go and Fuel to-go. The reason for this is that once the E6B is set to the correct ratio you can easily look at nil-wind time to go or fuel to go from the current waypoint and calculate the updated time to go or fuel to be used for the destination. This makes it very easy to estimate next waypoint or destination arrival times and fuel usage. Combined with the expected fuel tank reading provided with the flight plan, fuel in tanks at destination can be accurately calculated and alternates can be decided on based on the current reading and predicted fuel usage from current conditions.
Planes fly through the air faster at higher altitudes. There is no reason why you can't take this into account when you are working out your predicted new speed. If you don't then it will look like a little bit more tailwind when you do your calcs.
Most planes change direction after passing a wayoint and winds change along the track. So the ratio on the E6B set at the beginning of the trip will need to be updated. When changing direction you can expect a different amount of head/tail wind. You can calculate wind using the back of the E6B or you can estimate; whatever makes you happy. If the E6B is 100:133, 100Kts planned but actually 133Kts, then you can set the new ground speed (alt. compensated if you like) across from the planned speed, say 100:120, assuming we have less tailwind. You could also have decided that you were going to have 13Kts less tailwind and rotated the E6b back 13Kts to 100:120. Either way is OK.
The point to note is that the E6B requires small changes when updating calculations. Since it is always working on a ratio, the ratio will be similar when updating values; you would not expect to turn the outer ring too far. If you find yourself needing to turn the E6B a significant amount without a significant change in wind or throttle setting, then you may be making a mistake in your caclulations somewhere.
The flight plan shows to-go values as well as cumulative values. So at any waypoint you can see how many minutes, liters and miles to-go; but you can also see how many minutes, liters and miles have been traveled. If you are at a waypoint and realise you cannot make it to the destination and have to turn around, then the cumulative values now become your to-go values for the return trip.