Hi,
if you are prediabetic and on no medications, then you want to use the testing to see how high your blood sugar goes and also the effects of your lower carb diet and exercise on blood sugars.
Fasting blood sugar is a good baseline. Ideally fasting blood sugar is in the range of 70 - 95, with in the 80s considered perfect.
You want to then determine what is the maximum blood sugar you get after eating. This may range between 30 and 120 minutes after eating (though most drs will recommend testing between 90 - 120 minutes after starting eating). You can determine your personal testing time to catch your peak blood sugars by eating a regular meal and then testing every 30 minutes for say 2.5 hours. The peak blood sugar measured will then tell you your testing time. Thereafter you test at that time after eating. Ideal target peak post eating blood sugar is < 140 (and even better if it is < 120, or fairly close to your fasting level).
Work out which foods do and don't make you high using this method, and you can then adjust your diet accordingly. Most people find avoiding sugars and refine carbs (which can even include grains and fruits), will be the best approach.
Note: if you eat a high fat meal (pizza for example, the peak rise in blood sugar may be delayed). If you have something very sweet and without fibre such as fruit juice, your peak may occur earlier.
A 30 minute walk after a meal may be sufficient to bring sugars down to normal after eating as well, or at least help in getting them down.