From everything I have read here on YT it appears that adult weight is always a guessing game.
Some have said that by "rule of thumb" they should have had a "teapot" Yorkie. Their baby grew during its younger months and then stopped. Others have said that their Yorkie grew later and they have a "teapot" that they didn't expect. A guessing game at its best.
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Joanne
Eli
Chipper
Eva
Snowboy
Rosie(R.I.P. 1996-July 29, 2013)
Cocoa(R.I.P 1998-April 26, 2012):