I don't worry about it. Let the system ride to expected attenuation...be the beer, feel the beer...become a jedi master, if you will....then rack, and take FG. Multiple pulls with a thief waste the precious....future mature carbonated malt beverage IMO.
If you are really concerned about consistent FG over 3 days or stuck fermentation I would suggest getting a refractometer. It takes way less sample amount to get a reading and should give you that constant number you seek. Just adjust the Brix number to account for the alcohol. Google it. Drink a beer.
Keep it simple...a good thing to remember...It is homebrew after all, and should still taste better then ABInbev.
A wine thief is your best bet. Don't drill a glass carboy...
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