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: Sweet Potato Saison(
Seconary: Don't use one, generally
Kegged: Scottish Strong Ale
Planned: Either a Porter or an AIPA. Possibly a RIS.
"For a quart of ale is a dish for a king." - Shakespeare