Well, trial and error is the best way to go. Play on All-Pro. Play different match ups. Switch teams and play those match ups again. Note what the computer can and cannot do well consistently. Make changes accordingly. Make changes to user and computer abilities. I remember one thing I used to do was set computer pass blocking to 2 or 3 [or was it 20-30 on a 100 scale?] becasue until it was in the 2 or 3 range [2-30?] the computer would stone wall even elite user pass rushers.
Again, start on all-pro and make user/computer changes bit by bit until you find a set that give you a challenge with out cheating you. Also keep team talent in mind. If you are beating people up with the Bills or lions, you really ought to mess with the sliders until it is the other way around. if you can get to a point where user controled Lions feel like a chore to play with, but user controled Patriots or Colts
could win any game, you have done a fine job and are ready for a franchise mode. That way, building a good team will be noticeable and entertaining.