I look at the rockers also and adjust when a given valve is in the middle of the flat spot travel on the cam. As you turn the crank, it becomes readily apparent when a given lifter/valve is going over a "flat" or non eccentric part of the cam -just to rotate to be in the center of that and adjust.
For low rpm diesels there is almost no overlap in the cam; so you can frequently get away with adjusting the exhaust when the intake is at its fullest open point, then adjusting the intake when the exhaust is at the fullest open point for a given cylinder.