I am changing the reaction rolls for  the following reason.  Every other die roll but damage, one is the best thing you can roll.  Then when its time to roll a reaction,  20 is the best thing.  So to stay consistant, and to stop that confusing look my players give when  I explain that this is the one time they want to roll high,  I have changed the rule.

One rolls 1d20.  The better you roll UNDER your charisma, the better your result.  I use this mechanic for many things, so now its time to just use it for reactions.   

Example;   you have a charisma of 12.  You meet someone and I tell you to make a reaction roll.  You roll a 4.  12-4= 8.  Your success is eight.  I compare it to the existing roll and its an ok result.  You are neither loved nor hated.  Kind of blah.

So the reaction modifier is not needed as its taken care of by the stat.    You can still train in skills like wit, charm, seduction, conversationalist, etc... which would give your Charisma a boost by+2 for the die roll when the GM allows.