Nail cutting in Great Danes

When cutting Great Danes nails, is there a difference that cutting any other dogs nails? They are so huge now, that she is over a year old, I am kind of wondering about how short to go. To even with the pad of her foot when standing or shorter? I have just been cutting the tips until I take her to the vet, because they are all black, but he says they should be a little shorter. I am just really afraid of going too short. Is there a way to explain this to me that will make sense?
   
Thank you!