Just off the top of my head -- minor modification of your code to remove macro expansion:

oGrid = EVALUATE("THIS.PARENT."+ALLTRIM(THISFORM.GridName))  && Get reference to grid

m.MouseXCoord = MCOL(WONTOP(),3)
m.MouseYCoord = MROW(WONTOP(),3)

m.GridRelativeRow = oGrid.RelativeRow
m.GridTop = oGrid.Top
m.GridHeaderHeight = oGrid.HeaderHeight
m.GridRowHeight = oGrid.RowHeight
m.CalculatedPosition = m.GridTop + m.GridHeaderHeight + ((m.GridRelativeRow * m.GridRowHeight)-(m.GridRowHeight/2))
oGrid = .NULL.