Adjusting pattern with decreasing stitch number

So I am working on my first sweater, the Baby Poonam:
I am on the sleeves now which are worked in texture pat but with a decrease at the beginning and end of the row every 1/2". Texture pat is as follows

row 1: k1, p3, k1 repeat to end
row 2: purl
row 3: p2, k1, p1 repeat to last stitch, then p1
row 4: purl

The pattern I am using has notes added to it by a ravelry user. She says that in order to stay in texture pat while doing the sleeves you need to make sure each group of p3 is centered over the previous rows k1. This note being added makes sense because of course the decreasing number of stitches will throw off the pattern if you simply follow the pattern instructions above, the rows won’t line up correctly. My issue is that I now don’t know how to start my rows as a result. I mean I’m basically following a k1 p3 pattern so I could simply find (on paper) the first place a p3 and k1 will overlap, make sure the k1 is centered, and work backwards in that k1 p3 pattern to figure out how to start. My only issue there is I won’t always start the row with the right stitch (I’ll sometimes start row 1 with a purl or row 3 with a knit), is that okay?

You have exactly the right idea. Find a pattern area and count or name the sts backwards to the first stitch to figure out which stitch to begin with. It won’t matter how you start the row as long as the overall stitch pattern is maintained. This is called reading your knitting and is a useful skill to have.