There is really only one way to find her exact I:C ratio. But first, you need to know her correction factor. Heres how you test:
Find a time where she’s been fairly steady at about 150-180.
Give one unit of Novolog.
Test every half hour for 3 hours. Note the total drop in points. Lets say its 40. You also will be able to see how quickly it starts working (this is helpful info when giving boluses).
Next, you can test her I:C ratio. You will need a small amount of fast acting glucose, no fat or protein i.e. apple or orange juice, a bag of fruit snacks, etc.
make sure to read the label on the juice. If it says 6oz is one serving, take just that. Dont guess.
Take the juice and test every 30 minutes for 1.5-2 hours. Notice the rise at the end. Lets say she goes from 150 to 210. Lets say juice says carbs is 28g.
Now, you know how much one unit will drop her - 40. If shes now at 210, to correct back to 150, thats 60 points. 60/40=1.5 So now you know 1.5u will cover 28g, or to simplify, 1u will cover 19g of carbs, rounded up.
Keep in mind also its common I:C ratios can differ at different times of the day, so you’ll want to test like in the morning and maybe right before dinner.