I use 1 unit which brings me down by 3mmols when giving a correction dose. Sometimes it takes a good few hours for my sugars to come down though. I think my main issue is the times I eat out and have to 'guess' how much insulin to give myself. I also thought sweet potatoes were good so started eating them recently by baking in the oven as fries but they sent my blood sugars haywire!
Get hold of a copy of 'Think like a pancreas', and obviously read it LOL.
Test more (?) and keep records, on paper or a phone app and review them regularly.
Do a basal test first to make sure your slow acting insulin keeps you on an even keel, then you can check out your bolus and correction ratios after.
Exercise.
Don't eat out so much?! I'm totally with you on the eating out front, but sometimes it's a minefield of carbs and hidden sugars, it can be done using an 'educated guess' or repeated trial and error!
For me, a moderate carb intake helps to control my BG. I normally eat somewhere between 130-250g per day, and it allows for tighter control by minimising spikes using less insulin.
Another absolute champ of a technique is to pre-bolus. Basically, injecting your rapid acting insulin before you eat. Sometimes well before, sometimes shortly before. Really it depends on the fat content and glycemic load of the food you're eating. As everyone is different, it's really a personal discovery on a trial and error basis. But once you know what works, you can get some really flat BG profiles. It's detailed pretty well in the book that @slip has mentioned. You should exercise caution when using the pre-bolus technique though as you may hypo if you misjudge the GI of the food you're eating.
It's available in digital format on Amazon for under £8. So you can load it on your kindle or smartphone which I think is pretty good.
I second the advance bolusing. It helps enormously with spikes. Exercise can also help. Even a brief walk or housework or anything to keep active after a meal can make a surprising difference.
Testing lots is key to spotting when your BS sneaks up. Correcting in good time also helps.
If you eat out, maybe try to choose chain restaurants that have the carb count online? Or else choose meals that you can count easily eg something with boiled potatoes rather than a big bowl of pasta with an unguessable amount of carbs.
Testing during the night is good too so that you can spot any hidden highs there. Doing a basal test is also good to make sure that's correct. If it is, then move on to your meal ratios and see if any of those need tweaking.
My last hba1c was 5.8. I have always tested a LOT but now I also test 1 hour after eating( to catch any spike so it is brief) and correct then so that I will be in 7's range by 2 hours after meals .I have become more brave by trying to get down to 4's and 5's before meals though I used to be 5's and 6's..although tighter control means more lows ..