Hi
JacquiMorrison, It's great that you now have a Libre. I suggest that you use it to do some simple but useful experiments.
1. Choose a time when the graph on the Libre has been steady, level & not too high for a while & you are not doing any exercise & haven't eaten recently. Eat a small amount of carbohydrate - I'd use a jelly baby which is about 5gm, a glucose tablet is about 3gm I think.
Then sit back & watch what happens with the graph. After a little while it will start to rise - note the time & the bg reading. It will then climb for some time then slevel off. Keep watching until you are sure it is steady again then make a note of what the reading is and the time it levelled off.
You now know how much one jelly baby (or glucose tablet) raises your blood glucose. You also know how long before it is before takes effect*, & how long before your body has finished absorbing it. And all whilst sitting around doing nothing!
2. Again choose a time when the graph on the Libre has been steady, level for a while & you are not doing any exercise & haven't eaten recently.
You don't want it to be too low - say over 8, check with a blood test. Take 1 unit of insulin.
Back to the exhausting sitting around doing nothing, nothing energetic that is. Don't eat anything. Watch the graph. Be patient, it takes longer than you might think for insulin to do anything. In my case about an hour if I'm just sitting around. Again note when your bg starts to drop, for how long & when it steadies. Keep a close eye on it just in case it drops you too far - it shouldn't but it's still wise to have your usual hypo treatment available.
You now know how much 1 unit reduces your blood sugar, & how long before it takes effect.*
Repeat the experiments art different times of day.
The fact that insulin takes a lot longer to take effect than food does is the reason why people pre bolus before a meal
*Actually the Libre reading is a little delayed (10 - 15minutes?)