How exact does HCG doubling have to be?

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Just curious if doubling every two days is more of a general ballpark guideline or more like a bare minimum? When I see people post numbers it often seems like they are more than doubling.

Mine went from 16 to 63 in 4 days and doubling twice would have been 64 plus the second test was later in the day so I feel like it should have been slightly higher than 64?

Does anyone have experience with these kind of numbers? The doctor seemed happy with it but I’m still scared to be optimistic...