Open pit DC?

[Mercury]

Just a quick question, if there is an open pit, 60 feet deep, enclosed in an area of magical darkness, what should the reflex save DC to grab the edge of the floor and not fall? I do not have my DMG on me, and I am trying to prep an adventure. Thanks for your help.

Mercury

 

[da chicken]

The reflex save for all pit traps is DC 20.