Isn't this one of those questions to do with convergence between two given Lat/Longs, so therefore you need to find out at which point they want you to give the GCTrk value? (i.e. mid point, finish?) Trying to recall from Gen Nav, but its been a while.
Anyhow, convergence = change longitude x sin (mean lat) but then you have to figure out which way to apply the calculated value, i.e. a positive or a negative on the original trk. Bristol gave you a neat way of sketching it out and measuring it. I'll go back to the manuals and check it out, unless someone beats me to it (which I hope they will...yawn)...