I hear you, some people have been on the airplane for decades and still don't know how to use the mcdu to their advantage.
once on vectors it is most helpful to have track miles to touchdown to calculate an immaculate descent, and this is achieved by doing what goldenrivet suggests, though having PPOS will not give you accurate track miles, I'd have a From-To leg on the final approach course making sure the TO waypoint will be overflown (FAF) so typically ill have the FAF as the To way point and anything else on the centerline right behind it, you can also achieve this with a radial TO intercept to the FAF or equivalent.
just copy the active before you do this if you need to recall what was there.
and always ready to learn something new.