In our group we have found that the MI effects, such as spiders pouring out of wounds or weapons becoming snakes, is descriptive only and the actual game intent is to stop some course of action like swinging a sword, casting spells, or fighting all together. So the visual effect isnt as important as its intended affect.
With that in mind, we equate the levels of effect to translate into specific reductions in actions. A roll that equals Ego + 0 can be a loss of a half action, Ego +10 is a full action loss and -1 or -2 to DCV, depending on how clever the illusion or how it may play on the target's fears, etc. Note, if the purpose of the illusion is to get the target to move or avoid an area then the loss of actions is replaced with change in movement modifiers.
The Limitation of No Control can reflect that the Illusion pulls out the target's fear to change his or her actions, saving the conflict between GM and player of deciding how to interpret the end result of the illusion. But if the player like the spider theme, then agreeing on ahead of time what the the end goal of the illusion help both side play more smoothly by avoiding arguments of what should happen.