It might be a bit more complicated than that. The question actually asks for the cross track distance between the great circle track and the straight line track. You have answered for the cross track distance between the rhumb line and straight line tracks.
Assuming the question means what it says, on a polar chart great circles are very nearly straight lines, the difference arising from the difference between earth and chart convergency. Chart convergency here is 20º, earth convergency 17.3º, the difference is 2.7º. The angle between the great circle and straight line at each end of the track will be roughly half that, about 1.3º. The rhumb line track is about 600NM long, the great circle distance to half way would be less than half, guess at 290NM, the cross track distance is about 290NM x sin1.3º = 6.5NM, bodge it down a bit because we’re dealing with a 3D surface, I’d guess about 3 - 4NM.
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