Is there a formal order to declaring what you're using and what you're using for bonuses?
With magic use you don't want to go over your threshold, but you do want to maximize your chances to cast. It's a fine juggling act and often you have the option to channel or add Augments (Fey Sight). It seems sneaky for an opponent to decide to go with whatever would subtract less just to get you to go over your threshold. For that matter, you couldn't you just decide not to use shields or other bonuses you get as well?