Track closure rate at 90 kts with a 40 deg SCA is 90 sin 40. In 4 minutes, XTK error would have reduced by (4/60) x 90 sin 40 nmiles, i.e. 3.8567 miles. He should have been only .1433 miles (871 feet) off track after following the 40 deg SCA for 4 minutes - hence I can only assume that he didn't model the problem correctly...