Don't over complicate things. And don't try and learn something new at this stage.
1. Draw diversion route on your map
2. Measure the distance with the end of your thumb (you should know what that is - mine is 9nm)
3. Assess the track by sight
4. Pick a sensible heading based on the legs you have just flown
5. Assess your ground speed based on the legs you have just flown
6. Calculate ETA
7. Make a sensible heading adjustment at mid-point (if required)
That is more than adequate for a PPL diversion