Well spoken.  You (OP) could also try this:
Roll one save for your defenders.  Compare it to the spell DC.  For every point of save success, 5% more (than 50%) of your defenders succeed (assuming a fairly homogenous group of defenders).  For every point of failure, 5% less than 50 fail.
Joe PC casts Turn Green on 3 goblins (the black kind, not the green kind).  His spell DC is 13.  The goblins get +1 to their saves.  The GM rolls 15, total 16.  Since 16 is 3 points over 13, the GM adds 5% for each point (or 15%) to 50%.  That's 65%, or fudgingly, 66.66%.  Two of the goblins (66% of 3) succeed on their saves.
Mathematically flawless?  No.  
Time-saving?  Hopefully.
True to the 5e rules?  Beats me, the rules aren't out yet.