Mathematically Challenged

I had ordered this pattern for the Baby Alpaca Grande yarn I happen to have coveted, er, I mean bought. But I decided the pattern would be perfect for the Manos, with a few modifications. So I swatched AGAIN, and it turns out the Manos is 4 stitches to the inch. The Alpaca Grande is, and incidentally the pattern calls for, 3 stitches to the inch.

Now, I am mathematically challenged. I was able to figure out how many stitches to cast on to achieve the right size in inches. That was a miracle in and of itself. I have NOT been able to figure out how many stitches to decrease, and on which rows, to maintain the pattern. I'm only on the 2nd row, so this is not yet critical, especially since I'm planning on extending the length. If you, Dear Reader, are familiar with either the magical mathematical formulae or a source that explains the magic to achieve this miracle, kindly impart it to me. Also, if anyone is aware of an invisible decrease, I'd appreciate that information, too.