At least 50% should be. You'll find that genetics are not as clearcut as some make it seem. There are varying degrees at which the mottling will show. On some it might be three or four feathers tipped in white; on others it will be such that you'll never know one or the parents was black.
I have a frizzle polish rooster that is 1/2 tolblunt 1/2 golden laced. I got him to go with my polish hens for a mixed color polish flock. I thought about putting him with a spangled/mottled game hen to see how many mottled chicks I get.