If you're 4 miles left of track, turn right through by your Standard Closing Angle (approx. 60/IAS in miles per minute, i.e. 40 deg at 90KIAS), hold for 4 minutes, then reverse back onto your planned heading. Resynch DI and check the ac is in balance. Adjust your ETA by 1/3 of the 4 minutes, i.e. 1.3 mins if you used a 40 deg SCA.
If you're 2 miles right of track, turn left by SCA, hold for 2 minutes, reverse onto heading, resynch and check. Delay ETA by 2/3 of a minute.
Throw away the hard maths, just use Standard Closing Angle. Trouble is, it's so simple that the dinosaurs don't believe it and continue to force you to suffer hard and pointless mental calculations! SCA is a refinement of the ancient 1 in 60; if it's good enough for the RAF it's good enough for me!