Hi Ryomaden,
Completely agree with Francesca about assessing whether you need a smaller size or a SBA on the top. The easiest way to establish this is to take your high bust measurement, and pick the size where the bust matches that. So for example, using the McCalls sizes
here, if your high bust is 32.5", go with a size 10 all the way with an SBA. If your high bust is 30.5" you are right, you do need a 6, and may need to grade out to the waist and hips.
This Bluprint link is quite useful.
That said, I see that the finished bust, waist, hips measurements are on the pattern and there is (in my opinion) quite a lot of ease built in for what looks like quite a fitted dress, so bare that in mind. If you are a standard size 10 34.5" at the hip, an 8 would still give you 2" of ease there, which would be enough for me in a dress of this style. A 6 would only have 1" ease, which would definitely be a bit tight for me, but depending on how tight you like your clothes and what fabric you are using, you might actually be happy with that. Compare the size measurements in the link (or on the packet) with the finished measurements on the pattern packet to see how much ease there is. It might be useful to measure a ready to wear dress of a similar style if you have one where you like the fit.
If you do decide you need it, grading is not difficult. I haven't read it, but the pictures in
this Cashmerette article seem to demonstrate it well. I would grade between the notches and the lower 'lengthen or shorten' line in this case I think.
As it's your first pattern, I would recommend starting with a 'wearable toile' in a not-too expensive fabric, so that you can both practice techniques and check the fit before moving onto pricier cloth, but of course that is entirely up to you.