Hi, @Zohairraza! Welcome. 
While this is true, 40 is well over a 30% difference. According to a call I made for a failed sensor replacement, the 30/30 rule goes as follows: if your blood glucose meter reads less than 70, your CGM is supposed to be within 30 points of your BG. If your blood glucose meter reads 70 or above, your CGM Is supposed to read within 30% of your BG meter, meaning your sensor should’ve read 63 or higher. I just might be the pickiest sensor person around, and I have to say I rarely have to apply these rules to anything I see. Did they give you a hard time about replacing it?? For I think they would’ve been wrong in so doing.