Calculating free Ca2++ with EGTA buffers

[Fergus_Doherty at vme.ccc.nottingham.ac.uk]

Can anyone help me to calculate free Ca2++ in EGTA/CA buffers?  Basically 
I want to determine the effect of Ca on enzyme activity, and relatedly 
translocation of an enzyme to membranes-which is Ca -dependent.  I need 
to include EGTA to completely inhibit the enzyme, and then add back Ca.
This must have been done thousands of times but I am completely 
bamboozeld by the books I've looked at.  What I need is a simple 
explanation and perhaps a program, spreadsheet or Hypercard stack 
for the calculations (so Mac, or PC Excel).  Thanks.
FErgus Doherty, Biochemistry, Nottingham University, UK.