Baston,
I am very impressed by your DR Navigation skills.
How many pilots do you know that fly heading to the precise 0.1 degree and fly a precise IAS.
Planned heading 360. Flown heading 359 to 001.
Expected drift 0. Actual drift 2S to 2P. And do remember you can't get this drift from Doppler or GPS because that is feeding the TANS and would defeat the object of the exercise. No, you will have to assess it yourself using wind lanes.
Distance between Bel and the Mull 60nms. (Feel free to adjust the figures)
I see an easy cross track error of 3 nms.
Planned IAS / TAS / GS 120kts. Coast in in 30 minutes
Flown IAS 117 to 123.
Temperature / Density / Instrument variation. Actual TAS 114 to 126
Slight W/V variation GS 110 to 130.
Coast in time now between minute 28 and 32. You are up to 4nms in error along track, and you have flown to realistic degrees of accuracy.
As Walter says, tell us your magic system for reducing these margins.