All your presumptions are correct. Bear in mind though that the distance between the tracks need to be wide enough apart to handle stock if necessary. As you can see from the plan the centre track on both the sector plate and the scenic section are dead straight and form a datum. If the sector plate is then swung so that the centre (straight) track is aligned with the position you want the next track to align with it, it will allow you to calculate the approach angle of the track on the scenic section. Once that has been laid you will then be able to calculate the curve needed for the next sector plate track to align correctly with it. I've attached a somewhat ropey sketch to hopefully show what I mean. The method works but does require both the scenic and sector plate sections to be laid together.