Gillespie,
In flight the easiest way is to use the inverted fraction rule
(this is based on the 1/60 rule but much easier to calculate in your head)
You are 60nm along a 90nm track -this is 1/3 of your track left to run
Invert the fraction and multiply it by your angle off (not distance)
4 x 3 /1 = 12 degrees left to correct
This works for any angle or distance along track