42B to 40FF is a huge difference in cup sizes.
Whilst I agree with the info Francesca posted about - some breasts being fuller on top, others being fuller underneath, some are more pointy, some are flatter etc, what bra fitting really comes down to is finding a wire size - even in non-wire styles - which fits around the root of your breast (the natural crease) and matching it to a comfortably firm underband size. Info about that
here.Bra wires sizes are based on a B cup. So the wire from a 42B would be the same as the wire in a 44A or a 40C, a 38D, a 36E and so on. Within the same style of bra the cups of each of those sizes should be identical. It may well be that you find that different brands/different styles fit better or worse than each other. Charts like
this one show the wire size/cup size groups.
The Sewing Chest has lots of actual size drawings of wires.
If you have an old bra you are willing to sacrifice (or repair afterwards) remove the wires, and hold them up against yourself. If the wire sits nicely against your chest wall it's the right size. If it digs in and doesn't lie against the chest wall properly it's too small, if there's room between it and the crease it's too big.
Once you have found the right size wire and matched it to get the cup size-letter any fitting tweeks should be doable on a toile by making very small changes in the curves of the pattern pieces. On the Marlborough bra pattern those tweeks could be done either on the power bar seam or on the seam between upper and lower cup pieces.