I'm glad you're keeping a log. That's a huge benefit!
I would also be sure you're measuring or weighing his food for consistency each time. For some dogs even several pieces of kibble can make a difference.
You mentioned some walks. Is there a certain time of the day you usually go? Timing can be really important for some dogs. If insulin is quite active, you can get a large drop in blood sugar. Or, if its fading, a large rise can occur. Now that you're home testing, you'll be able to see how exercise effects him.
Consistency really is key with most dogs. The more consistent you are with food, shot times, exercise, etc, the easier it can be to regulate.
It sounds like Dutch has a large swing in blood sugar from high to low point. If you are able to complete a curve on him, we'd love to see his numbers and offer help if we can.
I would say if he shows signs of going too low again, I would reduce his insulin a unit and see if that helps.
Take care,
Patty