Barred is crisp (mostly even) lines that come from slow feather allowing the barring (depigmented areas) to grow evenly.
Cuckoo is more diffuse and softer looking. Faster feathering causes the depigmented areas to offset producing a "cuckoo" instead of "barring."
If you are working with cuckoo rooster, cuckoo hens, and a black hen...it will be trial and error to see who produces the best cuckooing.
You can breed the cuckoo roo over the cuckoo hens and take the best. That actually may be your best plan as it will further double gene cuckoo roos and single gene cuckoo females.
I find the irregularities begin with hybrid colors...breeding a barred/cuckoo roo to a solid black, at least the first generation, as you then immediately go down to single cuckoo/barring.
However, I have heard the breed to black to crispen rule too...but I have not seen that thus far in my efforts working with barring...black just becomes dominant or my patterns diffuse.
So personally, my fastest route has been to breed pure cuckoo/barred roo to pure cuckoo/barred females. IF and when you begin to see poor coloring, you may decide to create a second, separate line of cuckoo roo over black hen, producing the single gene males and females, and then line breed back to the cuckoo roo to regain double gene males, single gene females.
I would avoid immediately mixing the lines of cuckoo/cuckoo to cuckoo/black as you will be fighting diluting your cuckoo genes.
Does that make sense? My experience again with non-Silkie breeds. (I keep lovely Silkie ladies as brooding hens, but I don't breed Silkies).
LofMc
FYI
Photo of barring:
Photo of cuckoo