Track Error
Your Rule of the 2X TE works fine. Here is an expansion that before you may even reach the point of finding out your off course, you could fix it early.
Sound like nothing an old 1:60 Rule couldn't fix.
This is rather a complex explanation, however from this you can design your own rule of thumb.
1:60 Rule states that for every 60nm in distance a steadily held 1 degree deviation in Aircraft heading will place you 1nm abeam your checkpoint.
So, reduced to its lowest useful term is; 1nm~1Deg~101'.
Ex.10nm~1Deg~1010' or 30nm~1Deg~3030'/0.5 nm or 60nm~1Deg~6060'/1nm
Now, for the Ex.10nm~1Deg~1010', if we open that 1Deg deviation to 10 then; 10nm~10Deg~10,100'/1.66nm.
From here we have a base to work with.
Lets say your flying along a leg 60nm long with only 3 check points, which means approximate 20nm checkpoints. 10nm you check the MAP/GPS and estimate a 1nm/6076' XTE........10nm X 101' = 1010'per Degree. How many degrees are in the XTE of 6076'...6076'/1010' = 6 degrees. 6 degree is your XTE, however 6 degrees corrected turn will Parallel your course, so double the XTE=12 to meet your 1st checkpoint of 20nm over head.
A Quick Ref would be:
XTE (converted to feet)
Distance flown X 101'
Triple or Quadruple will intersect the leg (wont get into the details), but account for loss of time and extra fuel burn.
Hope this helps and not confuse you more.