You’re on the right track with the swatch in your yarn. Once you know the sts/inch or sts/10cm and the size of the blanket that you would like to make you can calculate the number of sts to cast on. That number can be adjusted to the given multiple of 12 +2.
Be sure to make a large enough swatch so that you can measure over the middle 4 inches or 10cm.
Bernat® Blanket Perfect Phasing appears to be the name of the yarn. What is the name of the pattern? If you found it on the Yarnspirations site perhaps you would post the link.
Before you start a pattern is the time to over think and second guess. Your’re dong the right thing here. Hope you find fabulous yarn for your blanket!
I think working this in seperate colours will be easier to control to get the effect on the picture, it’s a good choice.
Usually we can share a link to where the pattern can be bought or found if it’s free, this doesn’t go against copyright and takes traffic to the designer or pattern site meaning possibly more sales so it’s not a bad thing.
When needing help on the wording of a pattern Usually just post a line or two, just enough to get help rather than large portions.
All good things to consider. The benefit of the Bernat striped yarn is that you will have fewer ends to weave in from the different colors. The color change with this yarn might occur mid row but that won’t detract and the overall effect will be of the gradient.