If you are refreshing cell A1 externally and want to perform the copy based on cell B1's calculation, then you can use the Calculate event macro.
Say we want the first value in A1 that exceeds 100 to be captured in C1. Once the first value has been captured, no other values above 100 will be copied.
In B1 we enter:
=AND(A1>100,C1="")
and then install this event macro in the worksheet code area:
Private Sub Worksheet_Calculate()
Dim B As Boolean
B = Range("B1").Value
If B Then
Application.EnableEvents = False
Range("C1").Value = Range("A1").Value
Application.EnableEvents = True
End If
End Sub