If it is always from 50 cells down from where the formula is put in,
then this should work:

Function sum50()

currentrow = ActiveCell.Row
currentcolumn = ActiveCell.Column
sumtotal = 0
Do Until IsEmpty(Cells(currentrow + 50, currentcolumn))
sumtotal = Cells(currentrow + 50, currentcolumn).Value + sumtotal
currentrow = currentrow + 50
Loop
sum50 = sumtotal
End Function