In reverse order, the online supplier is going to pretty much on the mark in their measurements.
As far as a percent deviation, I don't think there is a general rule of thumb. As you surmise, different malts will have different tolerances. Darker malts have smaller tolerances for deviation because their impact is greater.
Brody's Brew House
Primary: Spiced ale
Planned: Butternut squash & gala apple harvest ale; brown ale or porter; barley wine
In the fridge: Dogfish 60 minute; Imperial Stout