Actually, the probability of roan also depends on the horse's zygosity.
He is a chestnut, so he is homozygous for red, no chance of a black based foal from his side of the genes.
Homozygous = 100% chance of roan foal
Heterozygous = 50% chance of roan foal
Homozygous = 100% chance of a foal carrying agouti
Heterozygous = 50% chance of a foal carrying agouti
Agouti will not show on a red based horse so the only way you could guarantee a foal that exhibited agouti would be to breed a mare that was homozygous for a black base coat (bay, brown, buckskin, smoky black, grullo, bay dun, etc).