Well, mackerel and broccoli make a fab low carb meal, so I would expect a nice low reading from it. Definitely one to keep in your menu repertoire
And a very good start!
As a useful technique, which will add to your ability to gauge blood glucose control, try testing before eating, and then do the 2 hr test after.
This gives you a starting point with which to assess exactly what that particular food does to you.
Of course, it also doubles the cost of your testing programme, but hey, that is the way the low carb cookie crumbles, I am afraid
It is generally advised (here on the forum) that you are aiming for no more than a 2mmol/l rise from a meal.
Any more than that, and you would consider adjusting portion sizes, or changing the food choices.
In your mackerel example, a result of 4.9 strongly suggests that the food did NOT raise you by more than 2.
Brilliant result.