When you use that foal calculator, do not put "unknown" for anything. If you do, you get impossible results.
If you're not sure, pick "heterozygous" as that is more likely for things like the gray gene, dun, etc. For Agouti (Bay modifier) chose heterozygous Aa if your horse is bay. If you have a chestnut in the mix, run it once with heterozygous and once with "non-agouti" to see what your chances are either way.
When you breed any gray horse to a non-gray horse, you either have a 50% chance of the foal going gray or a 100% chance, depending on the genetics of the gray parent, no matter what the base color of the foal ends up.