My mare is a gray, with a black base coat (from what I can tell) and my foal's sire was chestnut, so I had the same color choices as you. I ended up with a bay going-gray foal.
I think any time one parent is gray, you have at least a 50% chance of gray, no matter what the sire is.
I kind of look at it this way:
If the parents basecoat colors are black (my mare) and chestnut (the sire), then the color options for the foal are black, bay and chesntut.
Then, because the mare is also a gray, you have a 50% chance of gray.
Looking at it that way always made more sense to me than 16% this and 16% that.
So the foal will be either black, bay or chestnut (assuming her parents are any of those colors) with a 50% of turning gray.