How do I modify this pattern?

Hello friends!
I’m so grateful for forums like this as I’m stumped tonight.
I’ll be as specific as possible…
I’m learning to knit on this pattern (the tutorial video is linked below too) :

(which calls for a US size 50 for main front and back and a US size 17 for neck and cuffs)
I didn’t have the needle sizes nor the yarn, so I started knitting the back panel for the full 21 inches of the main back instruction on a US size 17 (12.5mm) needle and will do the collar and cuff in a smaller one to match it. | I also instead of casting on 28 stitches, cast on 78 for a wider fit and because it looked to be correct at that size instead for what I’m looking for.
My one question is how do I know how to modify the increase rows with how I’ve adjusted it so have a same or similar fit?
Again, from casting on, I changed needle size down and cast on 78 rows vs 28 rows since yarn was smaller and so were my main body needles.
This is her wonderful tutorial video:

So yes, that’s my question, now that I’m at the full 21 inches before moving on to increasing, I don’t know how to adjust the video instructions for how she is increasing. My yarn is similar size enough, so just wondering with my modification of needle and width, the math of it. x
She starts to talk about the change from the 21 inches into the increase stitch at the 13+ minute mark on the video.
I have read online how to quantify it with the math equations, but I’m sorry they don’t make sense to me yet =/

Hi and welcome to KH!
That’s quite a change in needle size. It’s a guess but you might try increasing 10sts across the row when you get to 21.5 inches. That would be about every 7sts. It’s worth a try to see what the look of the upper shoulder is. See how that works.