We're actually using matrix functions built into excel - no solver and no VB. It's three or four different pages of calculations. The first is the data, the second is the data sorted into the matrix, then we do the inverse matrix function, and output the final OPR values.
While it isn't the example I used to create our spreadsheet, this page will show you exactly how we go about it. Only the newer versions (2010 and newer) will allow large enough matrices to do the math.
http://www.duncanwil.co.uk/simult.html