Adding in-line pockets

I've added patch pockets to the Sleepover pants once, but I'd really prefer in-line pockets this go-round (matching Dad/son PJs for Father's Day). I'm planning to use the Art Museum pockets and am inclined to split the pants at the existing pattern notch. Or should I split the pattern closer to the pants front instead? Since the whole waistband is gathered I'm having a tough time figuring out how to measure where the side seam would be. Any ideas? Or am I overthinking this, and splitting them in half will be fine?