Dexcom user here since the G5 and currently using the G7 10 day sensor. celia16 comments are valid. A great feature of Dexcom CGM's is they can be calibrated unlike some other CGM's. Best calibration technique is to have a horizontal arrow➡️ for the last 15 minutes, meaning your glucose is stable. Glucose meters are not 100% accurate either. Some people will take the average of 3 finger stick readings and use that number to calibrate the CGM. This will link you to the Dexcom calibration chart: (It's labeled G6 but it's the same for the G7). https://cdn.media.amplience.net/i/dexcom/en-us-g6-meter-g6-comparison-table

Seems like 20% is a large variation for being in spec but that is what it is. Also note that CGM manufacturers all say that if the reading you see doesn't match the way you feel, e.g. CGM reads 115 and you sweaty, shaky, signs you are low, do a finger stick to verify the reading.

First 12-24 hours of a new CGM insertion, the readings can be a little woky. Your body reacting to a new object invasion, trauma in healing the wound and who knows what all, until the body settles down. If readings are off by 40%, I would calibrate it during the first 24 hours. For your comfort, do a finger stick occasionally, compare it to the chart and calibrate if needed.

Recommended placement of the CGM is the upper back of the arms. I get good readings there but I need the G7 to talk to my pump and best communication is accomplished when the two are in line of sight with each other. Holds true for your phone, or receiver as well. Don't be afraid to try other locations. Abdomen, upper chest, lower back, thighs are used. The interstitial fluid is all over your body. Also prolong pressure on the sensor, like laying on it when sleeping, will cause a sharp drop in the reading and may trigger a low glucose alert. It's a pain at night I know, don't panic and down glucose tables, or juice as the first thought. Look at the glucose reading chart on your phone or receiver and see if it did drop off suddenly. If in doubt, do a finger stick. That panic glucose treatment will just result in a high spike within 20 minutes or so and then you have to deal with that.

If you discover the sensor is flaky, call Dexcom. They have always replace "faulty" sensors even when the sensor is not on the back of the arm as recommended. A flat out sensor failure, rare for me, a form can be filled out on-line or in the Dexcom sensor app on your phone, to get a replacement.

I'd consider yourself fortunate that the sensor stays on so securely to your skin. Way better then it falling off early as some users experience. I too, need to literally rid it off. celia16's suggestions for removal are good.

Last bit of wisdom I'll pass on: Diabetes does not play by any rules. The same food intake with same insulin dosage that worked for several days with nice glucose readings, can cause a crash or high readings the next. It is said that there are 50+ variables that affect glucose levels. Some known to you (miscounted for the lasagna) and many you have no idea why, even the weather. And it is worse for women. Give yourself some grace. It's the prolonged uncontrolled highs that cause the harm. Of course, the window for lows are more urgent to deal with. All insulin dependent diabetics have bad days, and don't let anyone tell you otherwise. Keeping the numbers between the lines is the destination. It's figuring out the path, that constantly is changing, is the challenge. Day at a time. Good luck to you.