I am wondering if you are using the correct units. 0.64 megabytes is nothing. 0.64 x 30 = 19 megabytes. We used between 8 and 9 gigabytes last February in Florida. (We are grandfathered with Verizon for unlimited data.) Most plans give you 2 gigabytes. (mg is milligrams used mostly for medication dosage in the US)
If your leakage is 0.64 megabytes and you are using 3 to 4 Gigabytes a month, I wouldn't give it a second thought.
If the leakage is 0.64 gigabytes, shut your computer off and see if the leakage continues, if it does call Verizon. It may be the Verizon hotspot unit itself.
Paul