how do i get the position of a row in a grid to send to a popup screen to fit right under the row.

You don't need the row number. What you are looking for is OBJTOCLIENT(). You can use it to determine where to pop up your form relative to the selected cell in the grid.

Here is some code that I use to pop up a calendar underneath a textbox in a container. Should give you a good start:

*** Calculate where the popup calendar should be instantiated
*** So it pops up directly below the date text box
*** SYSMETRIC( 9 ) is the height of the Form's title bar in case you were curious
lnTop = OBJTOCLIENT( Thisform, 1 ) + OBJTOCLIENT( This, 1 ) + This.Height + ;
              IIF( Thisform.TitleBar = 1, SYSMETRIC( 9 ) + 2, 2 )
lnLeft = OBJTOCLIENT( Thisform, 2 ) + OBJTOCLIENT( This, 2 )

DO FORM Calendar WITH lnTop, lnLeft, This.txtDate.Value TO luValue