Re: black hole

     That's a good question.  There are of course different theories as to 
how one might determine such a quantity specifically for such an exotic 
object as a black hole.  Some complex and some of them simple.  I will 
direct you towards some straight forward answers as best I can.
     First, I can refer you to two books, which discuss several theories on 
black hole characteristics and some of the deeper questions which 
prevail.  The two books touch on entropy, but I can tell you that they have 
no simple equations or formulas for such a calculation, but, nonetheless, 
could answer some of your questions and lead you in the right direction.  
The two books of which I speak are :

                    "A Brief History of Time:  From Big Bangs to Black     
                      Holes", by Stephen Hawking.

                    "Cauldrons in the Cosmos:  Nuclear Astrophysics", by
                      Claus E. Rolfs and William S. Rodney.

     Having read both books, I found them to be very descriptive on several 
characteristics of stellar phenomena such as black holes, but are lacking 
in quantitative approaches of which you seem to seek.
     I also performed an in-depth search on the World Wide Web to find 
documentation or clarification on entropy determination for black holes.  I 
began my search by looking for a scientist by the name of David Ward, since 
I had read some things he had to say about thermodynamics and black holes 
and found an excellent result which fits your request very well and should 
provide you with a very clear answer which provides you with explanations 
and a nice smooth guide through the calculation.  The explanation is 
provided at the web site:
             
           members.aol.com/rebeccawrd/index.html

                 and is entitled: "Black Holes and Thermodynamics: The     
                                   Physics of Black Holes by David Ward"

     This explanation follows in the logic of there being a relation 
between the area of the event horizon and the total entropy of the black 
hole, which is explained in the Hawking book shown above.  The theory goes 
that as an object falls into a black hole, the entropy of the surroundings 
goes down and the entropy of the black hole increases.  This increase is 
thought to result in an increase in the area of the event horizon.  The 
explanation provided by David Ward encompasses this and touches on several 
other useful concepts in the calculation, such as black-body radiation and 
the Schwartzschild radius, which are needed to arrive at the resulting 
entropy of the black hole in question.

     Well, I hope these resources guide you to your answer.  Good luck.