Complex sentence: Senior officers then changed the calculated figures according to their whim: about half the estimates were adjusted and adjustments of 30-40 per cent were common; changes of 60-70 per cent were not unknown.
Split into simpler sentences: Senior officers then changed the calculated figures according to their whim. About half the estimates were adjusted; adjustments of 30% to 40% were common, and changes of 60% to 70% were not unknown.