Some moms will say "if they're crying the need more sleep, if they're happy get them up." My baby wakes from nearly all naps exactly the same way, so I couldn't go by her mood. At 18 weeks we were extending all naps to 2 hours or as close to 2 hours as possible. Now our naps have settled around 1 hour 40 minutes without intervention and I'm happy with that. Not all babies will do 2 2-hour naps, some are fine with 1.5 hours. Keep in mind the longer he sleeps before waking, the more difficult it might be to extend as he's had more sleep and is less inclined to go back down.
If he sleeps for less than 1.5 hours, you can adjust your routine by moving bedtime up 1/2 hour or so to compensate. Or he may do fine with a longer A time right before bed.
You're doing a great job! The schedules in the book are only for guidance, you know your lo best.