The trick is to know how many points you are low and calaculate how many grams of glucose you need and eat only that much.
If you are at 60, and you want to go to 80, that's a difference of 20 mg/dl. If you know that each gram of carb raises your BG 5 points, then eating exactly 1 4 gram tablet should get you there. If you are at 50 and want to go to 100, then 50/5 = 10 grams so eat 2 and 1/2 tablets (they have a line on them to split them, usually).
The exception is when you have LOTS of insulin on board because you massively overdosed insulin or underestimated what you ate. Then this sort of calculation won't work as you need to offset all the insulin that is still active.
A quick way to estimate how much 1 gram of carb wil raise your BG is to divide
your ISF by your I:C ratio.
Let's say your ISF is 50. 1u of insulin lowers your BG by 50 mg/dl.
Let's say your I:C ratio is 1:10 1U of insulin covers 10grams of carb.
50mg/dl / 10 g = 5 mg/dl per gram. Each gram of carb will raise your bg 5 points.
So a single 4 g tablet will raise you 20 points.
This estimate should be tested of course , by seeing what happens when you eat one tablet when your BG is at a known value and stable.