> What I have noticed is that they cut far less off than I would think needs to be cut off

I'd like to think that based on lots more experience they know better than I do where the quick is.  When I look at my dog's nails I think that they are probably about twice as long as they should be.  This is how you end up making a bloody mess on the floor and traumatizing your dog as far as the cutters are concerned.  It's better left to the professionals methinks.