Buy a handheld GPS.
or .... draw lines at 10 deg either side of track starting from the start and back from the destination. Fan lines someone has called them earlier.
When you have fixed your position as being halfway between your desired track and the 10 deg line you have drawn from the start point, that is 5 deg off track, but the same point might only be 1/3 of the way between your track line and the 10 deg line from the destination, call that 3 deg.
If you only turn 5 deg toward track you will only parallel, but if you add the 5 and the 3 and turn 8 deg. towards track - Bingo ! Works for any distance along track, not much maths involved.
If I knew how to post diagrams to PPRuNe I'd show you.
The real trick is not to try to do a maths exam in a rattling steel cabinet, big numbers, commonsense
( Oh! and buy that GPS ! )
Used to navigate 707's with a sextant, recently flew a microlight into controlled airspace, needed a second bearing to make my report to ATC, couldn't be bothered re-programming the panel mounted GPS, pulled a handheld from my pocket - had to laugh, sextants in 707's, now 2 GPS's in a microlight !!
Use the KISS method. Keep It Simple, Stupid.