I've never liked any of the rules I've seen due to the varying nature of spells, or poisons, or whatever reason you are trying for a touch.
For example, do you just need to touch the character in general (i.e. armor, cloths, etc) or do you need to actually touch their skin and if you need to touch their skin what if they are wearing AT20 (Full Plate Mail) and technically do not have any exposed skin?
Just touching someone with no armor on that is trying to evade you might be difficult. Just touching someone in AT20 would be very easy, but if you need to touch their SKIN it might be vastly more difficult. Also, are you just going to rush them and go for an all out bear hug disregarding defensive thoughts or are you going to just try for a precision fingertip tap?
It really just comes down to the situation if you ask me. If I were to put an official rule to it I'd simply judge the difficulty based on the various factors and have them making a moving maneuver roll based on that difficulty rating with the targets Qu bonus and the attackers Ag bonus subtracted/added in.