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While I don't know the exact range that AGA allows for, I do know two things to keep in mind.
The first is that the sire is not a perfect foundation, just a dark red dun create (or technically a clone of). That means there was no technical guarantee to the sire's quality, other than that he passed BA (or possibly SBA). That allows for him to be a lower C than a PF (which is the highest C at 100% breeding ability).
The second is that AGA probably allows for roughly one or two percentage points in either direction, so if yours is at the very lowest end of that, assuming two points variance, it could potentially take a full 5% to bring him up to StS, or if by chance it's 3% in either direction you may not get it even with a 5% boost.
I doubt it's more than 2%, though, and may possibly be only 1% either way, as C to B to A paper goes from 100%> C, 101-109% B, 110%< A. You can check out Ammit's old horses from a pasture breeding experiment she did with different levels of boosting to see the paperings on them.

I'll be watching this thread to see if anyone who has more experience can speak up on how far AGA allows, because I am interested too.

I do not know if percent breeding ability uses decimals or not, and I haven't seen anything to indicate such.

For memory, I think the "as good as" range is 4 points (or percent), I can't remember why I think that though! but it means if they are the worst they can be and still be aga, then the 5% boost will bring them up to superior. A 1-4% boost would only have them still aga. My assumptions come from boosting a lot of horses, and works for me! But may not be technically correct!