Ending Macro - Variable Range

The following macro pulls calculated data from another worksheet and then performs a calculation based on that copied over data, then moves down a row and repeats. It is currently set-up to go down 300 rows. I want it to stop after it hits an empty cell in column C, but have not been able to figure it out. Any help is appreciated.

Sub Backlog_Update()

'
' Backlog_Correction Macro
' Backlog Copy from International File.
'

'
Dim startRow As Integer
startRow = 2
endRow = 300

Do
    Windows("Daily_DSI_Report.xlsm").Activate
    Sheets("LCD").Select
    Range("A" & startRow).Select
    Selection.Copy
    Windows("DASHBOARDS-REPORT.XLS").Activate
    Range("T1").Select
    ActiveSheet.Paste
    Range("V2").Select
    Application.CutCopyMode = False
    Selection.Copy
    Windows("Daily_DSI_Report.xlsm").Activate
    Range("AG" & startRow).Select
    Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
        :=False, Transpose:=False
    Range("B" & startRow).Select
    Application.CutCopyMode = False
    ActiveCell.FormulaR1C1 = "=IFERROR(IF(RC[1]-RC[31]<0,0,(RC[1]-RC[31])),0)"
    Range("A" & startRow).Select
    
    startRow = startRow + 1
    
Loop Until startRow > endRow
End Sub

Thank you for the amazingly quick response! How do I incorporate that into the macro? I tried replacing "endRow= 300" with your recommendation, and it just keeps going. I tried it with changing (Rows.Count,1) to (Rows.Count,3) thinking it would key on column C, but it just kept going. I tried a couple of Different "Loop Untils" but no luck. I'm still very green with macros, but I'll get there...

Check that.... I can make the macro work. Currently I have this as the third macro to be run in a set. The first one copies over the base data, removes duplicates and sorts. The second one just sets up the formula on the other worksheet, and then this one. If I run this one by itself, it works and stops at the right place. When I try and run the set, it just keeps going. What causes that?