Excel 2013 VBA add new line to Formula - vba

Question

With my VBA Code i add a new line to a table. In the last row of the table i have a =Sum(SumIf(...)) Formula, this row will jump one row down if i add the new line.
So now i want to add the added line to the Formula.
This is my Code so far:
Private Sub CommandButton1_Click()
Dim lastrow As Long
Dim i As Integer
lastrow = ActiveSheet.Cells(Rows.Count, "A").End(xlUp).Row + 1
i = 4
Do Until i = lastrow
calc = "SUMIF($K$3:$K$19,N" & i & ",$L$3:$L$19))"
ActiveSheet.Range("N" & lastrow + 1 & ":BI" & lastrow + 1).Formula = "=SUM(SUMIF($K$3:$K$19,N3,$L$3:$L$19)," & calc
i = i + 1
Loop
End Sub
In fact it works fine, but in the End the Formula looks like this:
=Sum(SumIf($K$3:$K$19,N3,$L$3:$L$19),SumIF($K$3:$K$19,N21,$L$3:$L$19))
What do i have to add to the code that every row will be added to the Formula?
Hope someone can help me. I am new to VBA.
EDIT:
The Worksheet shows all assignments and the working progress. With a CommandButton i add a new assignment to the table. So right now my table has 20 rows, in row 21 is the =SUM(SUMIF(..)) - Formula. If i add a new assignment the formula goes to row 22 and in row 21 the new assignment will be shown.
With my Code i want that every row will be added to the formula. Right now the formula goes from N3 to N20. If i add the new line the N21 will not be added to the formula. With my code above it only shows the first and the last row.
In fact i want that the formula will look like this:
=SUM(SUMIF($K$3:$K$19,N3,$L$3:$L$19),SUMIF($K$3:$K$19,N4,$L$3:$L$19) ... SUMIF($K$3:$K$19,N21,$L$3:$L$19))
This is my table hope it will help

Answer 1

This will solve your problem.
You need to assign a value to the variable "calc" before your loop. Then add to its value inside the loop with another statement. Finally, do not put the formula into the cells' values until after you have exited the loop and add a final parentheses to the formula.
Dim lastrow As Long
Dim i As Integer
lastrow = ActiveSheet.Cells(Rows.Count, "A").End(xlUp).Row + 1
i = 4
calc = "=SUM(SUMIF($K$3:$K$19,N3,$L$3:$L$19),"
Do Until i = lastrow
calc = calc & "SUMIF($K$3:$K$19,N" & i & ",$L$3:$L$19),"
i = i + 1
Loop
ActiveSheet.Range("N" & lastrow + 1 & ":BI" & lastrow + 1).Value = calc & ")"