There's just no way to tell...even at 8 weeks the weight isn't always accurate. I have three examples for you...
A) Chloe, my sister's dog, was 2.2 lbs at 6 weeks. Her weight chart showed her being 7 or 8 lbs full grown. At 12 weeks she was 3 lbs. At 16 weeks she was 3.5 lbs. At 6 months she was 4 lbs. And full grown she ended up being 5.5 lbs. A WHOLE different ballpark than 7 or 8. She wasn't underfed. She's very healthy...just stopped growing earlier.
B) Faith was on the growth chart at 5-6 lbs full grown when she was 8 weeks old. By 12 weeks she hadn't grown AT ALL since 8 weeks, neither had any of her brothers or sisters. She's now 2.5 lbs at 7 months, she'll MAYBE hit 3 lbs full grown. Her brothers and sisters are around the same. Again, they weren't malnourished...just hit their growth spurts early.
C) Sophie was predicted to be 5 lbs according to her growth chart at 12 weeks. By 4 months she was 3 lbs. Then from 4 months to 11 months (now) she only gained 1/2 a lb. She MAY make it to 3 3/4 lbs or possibly 4 full grown.
So there's really no way of knowing for sure...