ah why was I never told this...?
I think it's maybe a hangover from the days when faster insulins first came in.
The medical profession were both (a) enthusiastic about these wonderful new insulins, thinking, great, fast insulin - bolus at or after a meal and it'll all be ok, and (b) they're very conservative - docs and dsns seem to prefer the notion of us having spikes as long as we're ok by next meal as opposed to the risk of hypos if the pre-bolus timing is too long.
There's a bit of a paradigm shift going on at the moment now that libre/cgm is becoming more common than it was.
Whereas DAFNE tends to be, "don't test between meals unless you're hypo, and save corrections till meals", some hospitals are now actively promoting the notion of making small adjustments between meals if you've got libre/cgm.
As the other posts have said, pre-bolusing makes an incredible difference to post-meal spikes. It's a different mindset compared to strips alone and takes a bit of practice.
Here's an example.
I'll take a shot, I can then watch to see the cgm trace start to track down - that's an indication that the insulin is in stream and working. My assessment of it, in relation to how far I'll let it inflect down and to when I eat, might be influenced by what level I started at, whether it was trending up or down, and what I'm going to eat. I don't go anywhere near letting it rule my day though: I'll generally have an idea about how long a pre-bolus I'll need for a particular meal, and can sort things with an adjustment later on anyway, so it's not like I'm sitting watching a graph all the time, but it's a useful thing to pay attention to if you're just starting out with this sort of thing.
White rice spikes hard and fast for me, so I'd pre-bolus quite a bit for that, say, 20 mins, and it'd still be kinda unpredictable, so I generally avoid white rice unless I'm out at a Thai restaurant, (b'cos Thai green curry really doesn't work without white rice!). But brown rice, because of the fibre tends not to spike, so I'd pre-bolus about 10 mins as I'd likely start going too low with brown on 20. I'd also be adjusting the timings based on other things which would affect absorption, like how much fat etc is in the meal, and the portion size.
And then, the other thing I'd do, and this is the big difference from DAFNE, is that if I'm using cgm/libre, I'd be keeping an eye on how it's trending over the next hour or two.
If I started getting a sense that the graph trace is inflecting too sharply upwards, I wouldn't be slow to consider firing in a 1, 2 or 3u correction shot so that what might have turned into a serious spike will get pinned before it even happens.
Remember that we can never be entirely certain about our carb counts, nor how our digestion is panning out on that particular day, nor how much of the insulin in that bolus will actually end up working as opposed to getting killed by insulin antagonists before it does anything.
There's an inherent uncertainty in all of this. What cgm/libre brings to the game is the ability to see how it's panning out along the way in real time. That opens up the possibility of making small adjustments in real time to keep it in line, instead of a large adjustment at the next meal.
It takes practice but it's worth the effort. You can end up doing neat things like waiting till the trace starts inflecting down, so that the food is then playing catch up with the insulin, so you don't actually end up with a spike at all even on a 75 to 100g meal. But there is a risk of getting that wrong and going too low.
Stephen Ponder's book Sugar Surfing is a good read about this sort of thing, helped me a lot when I got libre.
Good luck, it's a brave new world you're in with libre!