estimating is after all, an estimate!
When I brought Ginger home I followed all the formulas I could find, asked the vet, etc. Came up with an estimate of 8-9 lbs.
My baby girl has topped out at 12.4 lbs (and she's not fat!). If I had estimated 12.4 lbs.
The lesson I learned...don't spend $150 on a carrier because it will probably be the right size...only time will truly tell!
On a side note, when I brought my shih-tzu home (this was about 18 years ago) at 6 mos. old she weighed just under 6 lbs. With 6 more months of "growing" to go, she never got any bigger than 6.5 sopping wet (and that was when she was a heavy middle aged dog, LOL).
Anyway, I tend to babble...
good luck with your baby, and enjoy every minute!