You get 2,5% less drops per level difference between you and the zone you are in, except the first 2 levels. So if you are 68 in Sarn (61), you will get (68 - 61) = 7 level difference. The first two levels dont count so you have 5 effective level difference. That means 12,5% less drops (forgot about the 2 free levels in my previous post).
However, for the purpose of this calculation, your level is never considered to be higher than 68 (because the original highest zone before changes was 66, not considering maps, so it was done so that you would never have any penalties mainly for map drops when farming the highest level zone available to you).
So whether you are 68 or 88, you will always have 12,5% penalty to drops in Sarn and zero penalty in Catacombs or Lunaris.