Does anyone know the criteria Excel uses to determine what order to calulate
cells in?

I have several workbooks which contain user-defined functions (contained in
an add-in) as formulas and can put a break-point in the add-in to see the
order in which the cells are calculated. These functions are only called when
Excel calculates the cell in which one appears and none of them depend on the
calculation of other cells.

However, the order of calulation Excel uses differs from workbook to workbook.

In one, it starts in the cell in the right-most column and the bottom-most
row and goes up the column to the top, then starts in the bottom-most cell of
the next column to the left.

In another, it starts in the cell in the top row of the right-most column
and goes along the row to the left and then when it has done the whole row,
drops down a row and starts again in the right-most cell.

I am trying make the user-defined function code more efficient and to know
what order Excel will calculate in would be immensely helpful.

Thanks,
Rosemary