Hello again @Gwynn . 🙂 Thank you for the link there, that's a nice looking shrug.
What I can say from my experience making, and wearing shrugs, is that the edges should not be overly tight as if they are, they will pull the whole garment in, which makes it uncomfortable.
Could you contact the designer to see what they might tell youi, or even LoveCraft them selves, they tend toward being very helpful.
Taking into account I only have an image of the pattern, and my experience with the fit of these garments, I can't see that a stretchy cast on would be a disadvantage. Using the 15mm needles to cast on would also be workable to get gauge.
I wouldn't worry about making the gauge a bit bigger with a shrug of this type, definitely avoid it being smaller though. If you measure yourself and compare your meaurements to the pattern size measurements, you would be able to determine how much wearer ease (the space between your body and where the garment fits it) you could have for comfort.
Also, keep the edges, and the cast off, from being too tight too - if there is a stretchy cast off that can be used, that would be useful also as it's the cast on and cast off edges that form to make the top and bottom edges, while the sides form the arm/ sleeve edges. I hope that makes sense.
I've made a couple of these in crochet, and found them to not be as confortable on my larger body as I would have liked them to be. I learned a lot about fit and gauge and general patterns by making them though.
I'dl love to keep hearing how you're going with it @Gwynn 
I've been thinking about your tension/ gauge swatch @Gwynn and thought I should check with you about a couple of things I forgot to include ... I'm not sure if you're a newish knitter or somewhat experienced so thought I'd include a few things for you, and for others who might be reading along.
I didn't check to make sure that the yarn you're using is the same yarn called for in the pattern, as this will make a difference to any pattern gauge. If it's not the same yarn, it would be best to use the needle size indicated on the yarn band, rather than the ones listed in the pattern as they are meant for the yarn the pattern lists. I rarely use the yarn a pattern calls for any more, my stash has been gathered more from the point of view of 'ooh, I want to make something for me out of THAT yarn', than from, "oh, I want to make that pattern' sort of thing. 
I break many, many so called rules with my crochet!!!
Also, and this is something I only learned a few years ago, 'cos swatching for gauge has always been something I'm not good with doing, I know how, just don't wanna, unless it's really important, then I will.
To make a properly usable gauge swatch, it needs to be roughly 6 stitches more than the required number of stitches asked for, AND 6 rows longer than the required number of rows asked for. Often, thos extra 3 stitches on the sides are done in a differnt stitch from the pattern you're using so the edges don't roll.
The reason it's done larger is to give a border around the area you're measuring and thus avoid improper measurements happening due to the edges being a bit distorted, and to be able to get a nice piece in the middle of that swatch to measure.
I hope that makes sense.
Then, the swatch needs to be washed and blocked in the manner you'll block the finished item. Blocking makes an enormous difference! I am a big fan of blocking my work!
You may find with blocking you already have gauge if you bind off the swatches and wash and block them!
If you want to reuse a yarn, and it's acrylic, be very careful about steam pressing it as you can 'melt' the yarn together and it stays that way for ever more. Used with care, steam blocking this way can be really useful for making things that will get roughly handled and machine washed and dried though.
Hoping this helps instead of hindering
@Judi9877I finally finished the first square of my Stallion blanket today! Unfortunately it's more of a parallelogram, because my tension changed from 17 cm down to 14.5 cm over the course of knitting the square. 😞 Mum says I'll have to undo it all and redo it if I want squares for a blanket.
Hi @Gwynn That's a pretty stitch pattern.
When I first saw it I thought, that might block out, but then I thought, maybe not if it's going to be joined to others for a blanket. I can see the differences in the stitches/ rows too, where the tension changed.
Oh, the number of things I've frogged (pulled undone) in order to get it right!
How is your other swatching going?
I have to find a new project myself.
Happy knitting!
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Help us push aside the stigma and discrimination surrounding complex mental health and change the way people talk about, and care for, mental illness.
SANE acknowledges the Traditional Owners of Country throughout Australia and recognises the continuing connection to lands, waters and communities. We pay our respect to Aboriginal and Torres Strait Islander cultures; and to Elders past and present.
SANE values diversity. We are committed to providing a safe, culturally appropriate, and inclusive service for all people, regardless of their ethnicity, faith, disability, sexuality, or gender identity.