Weir capacity is the easy part: Capacity = Cw * base width * water head^1.5

Cw varies with head and breadth of spillway.

Per the NRCS Engineering Field Manual, Chapter 11, For Grass-lined BC Weir: Cw for grass at 10-15 feet broad is 1.99 at 0.5 feet or less, to 2.8 at 2.4 feet.  

Cw for paved weirs can go to 3.33 over a broad-crest shape and 3.97 over an ogee shape; but find a reference to cite if you want to use anything over 3.1.  3.1 is fairly conservative and should be universally acceptable.

NOW for the hard part...make sure you have good (and extensive) hydrology for the flow you will be conveying.  You are far more likely to get the inflow hydrology wrong than the weir flow calc.  But the Army Corp will require a dam of this size to have H&H sealed and submitted by a P.E., so the hydrology will be covered with his or her seal.

contact your state department of water resources first and talk with the state dam safety engineer.  this agency may require a permit for your dam and either way, they can help you with the design requirements.  they may also recommend a qualified dam engineer to design it.  try the following website:  http://www.damsafety.org/

At 80 acre-feet of storage and 20 ft. height, you are definately geeting into more than the typical farm pond.  You need someone experience in dam design to help you out. You will probably also be getting into permitting issues, so the agency will probably require a P.E. certification.