In said example your VNAV path will continously recalculate itself based on the LNAV capture criteria, within 3 miles, or on an intercept course of less than 90 degrees. If you are within 3 miles on a downwind leg it will calculate a path based on the shortest LNAV track which will be an impossible intercept course and less track miles. Therefore you're better off either creating a point or putting CF on top rather than extending from CF. However getting vectors on to finals 15 miles out and more, extending from CF and selecting LNAV will intercept the final approach track (as you know, for sure) and also provides you with an accurate VNAV path.