You're not making a mess of things! You're still new to this, and it's very easy to feel overwhelmed, I know. You're trying very hard to do all the right things for your kitty. We all have those "Oops!" moments when we wonder why we did something and are afraid we made the wrong decision, you just learn to roll with it and try to remember not to do it again.
If you're only 3 hours away from his PM shot I would hold off on food until then, you want to make sure he eats before he gets his shot.
In a worst case scenario, if you have to use your human meter, folks here who also use a human meter may be able to help you understand those numbers, but I'm not familiar with the Aviva Accu-check so I don't want to swear to it. I'm guessing, though, that it's the same as any other human meter.
Keep us posted, please!