That is a big reduction in bg levels, perhaps injecting 20-30 mins before you eat will reduce the spike considerably further
Just a word to the wise, if the porridge you eat is the instant/quick cook variety like Readybrek then think about changing your porridge to a one that isn't as highly processed as this will have less of a impact on your bg levels, I use Jumbo or Pinhead oats but also add some fat to the mix in the way of seeds and natural greek yogurt, it adds a little more carbs to the meal but the fat content helps to reduce the spike which results in more stable postprandial bg levels,