The old (ie about 10 yrs ago) Australian Senior Commmercial/ATPL navigation exam used to require the calculation of off track ETPs.
This may have changed since the new syllabus was introduced.
As I recall, one method involved plotting + a degree of interpolation once in the approximately correct place. The wind's effect was included in the calculations.
Can't remember anymore.
Anyone with an old SCPL/ATPL textbook?