How to calculate for buttonhole placement

Hello-I’m working on a cardi pattern that is worked from the bottom up. The length from the bottom to the underarm is 12" and I want to make mine longer, so I end up with 16". The pattern is written as follows; “As written, cardigan measure 12” to underarm after blocking. If more length is desired, work more rows before beginning buttonhole sequence; buttonhole placement is calculated so that top buttonhole is placed close to the neckline." Previous to that note it says to work 17 rows in pattern, ending with a WS row. That’s where I stopped and after above note, it gives the buttonhole row. “Buttonhole Row (RS) K1, p2tog, yo, p1, k1, work in pattern to end. Work 15 rows in pattern. Repeat 16 rows 5 times more, then work Buttonhole Row once more.” This pattern calls for 7 buttons, I will of course need more button, but how many more? Can you please tell me out to calculate the placement of the buttonholes. Thank you!

I think that if you work until you’ve completed 9 buttonholes, you should end up with close to the length you want and still have the buttonholes line up correctly at the top and bottom.