Great, thanks.
It’s a Sirdar pattern but I couldn’t find a number.
The narrowing of the gown, from the fuller skirt at the bottom to the narrower skirt at the top, is being produced by using smaller and smaller needles rather than decreasing the stitch count. This sounds lovely as the lace pattern will keep continuity whilst also getting smaller for fit and design.
So, the bottom section on 5mm needles is going to stretch in width (86cm) more than the next section worked on 4.5mm needles stretches (80cm) and the third section, on 4mm needles, stretches less again (68cm).
The main thing here, I would say, is to be able to replicate this for the front. I am assuming that as this comes under “Back” there is a later section for “Front” and that it is going to tell you to work as for Back and later a section for "Finishing " or something similar which will say to seam the sides.
I may be wrong! Check the pattern.
I can imagine another way of making this would be to make the full skirt in one large section (rather than 2) and seam it in one place rather than two side seams. But at this point I don’t think this is how it’s worked. If you read ahead you can let us know.
If I was making this I would do a few things to ensure getting the front at back to match up, just suggestions.
I would keep a track of how many rows for each section on the Back and then work this same number of rows for the Front. Jotting down how many rows in each needle size.
I would place a removable marker or embroidery yarn loop on each end of the row where the needle size changes (ie on the first row of a new needle size). Later these markers can be matched at the long seams helping to keep on track and match up the sides well.
I may also place a marker yarn loop every 10 rows (these in a different colour to the needle change markers), again each end of the row. As it is a long gown these markers could really come in handy when seaming sides to match up rows and make the job easier.
Keeping track of the number of rows means not having to try to stretch and measure the fabric to know how many rows to to for the front. When seaming you want the same number ideally.
I like to use embroidery silks/threads for marking loops as they are thinner than the knitting yarn and I can even reuse the thread on another project. Thread up a blunt tapestry needle with the marker thread and thread it through the stitch on the needle. Then join the 2 ends with a slip knot so it can be undone.
Embroidery thread is also quite good for life lines as it is thin and slippery so doesn’t catch on the yarn.
Do you know how to use a life line? It would be well worth putting some in for this big project, lots of people use proactive lifelines for lace projects.