Hi Tina,
I'm not in the Broadcast world since many years, but I understand that if you are cutting RGB at 5% and 90%, you can't get any pure Black (R=G=B==0) neither pure White (R=G=B=100). On my knowledge, R,G and be must be cut at 0% and 100%.
Any way, I don't want to mislead you and I suggest you to post in the COW's "Broadcast Forum".
Rafael
http://www.nagavideo.com