Gidday RL,
If I may make a further comment to the extended center line, next time you are on base (90 degree intercept) and outside the active way-point, allow yourself to get a little low and level off momentarily and you will find the VNAV pointer will become stationary which will demonstrate it is not calculating track miles to the next way-point (In HEADING SELECT that is). Give it a try.
Just to clarify further, VNAV calculates profile in distance to the active way-point in LNAV or on the magenta line that is, which is why it is stationary when on a 90 degree intercept in HEADING SELECT.
And by the way, there is nothing wrong with knowing the magenta lines capability's as Mr Boeing (and others) put it there for us to use as a great tool for situational awareness as long as we use it wisely. As for the mark one eye ball I like to put say a 10 mile ring in the fix page off the runway threshold and use that as a 3000' mark and eyeball the white map range scale for extra miles. What ever smokes your tires and works on the day I reckon.
PS. It would be great if some one could quote a reference to this question's answer some where.
Cheers
