vba setting up starting cell to fill down formula

[jaik22]

lastRow44 = Cells(Rows.Count, "A").End(xlUp).Row

Range("A" & LastRow3).End(xlDown).Offset(0, 11).Formula = "=Sum(M50:M" & lastRow44 & ")"

I have vba code like this, but I want to modify it to replace M50:M to ("A" & LastRow3) to make code dynamic.
However If I replace it, it does not work. Is there any way to make starting cell dynamic?

Thank you!

 

[Joe4]

Please explain your data structure, and exactly what you are looking to do (in plain English).
Use screen images if it helps (see section B here: https://www.mrexcel.com/forum/board-announcements/127080-guidelines-forum-use.html)