Dermine Value Of Cell Number Preceeded With Text Character

I've tried and tried and I con not get it right. In A1 I have a validation, it onlu allows to enter a 6-digit number between 100000-899999 or the same numbers with the letter I in front, that is I100000-I899999. Now I need a macro to check that cell F2 is not empty if the value in Cell A1 is above 299999. And this should also apply if there is an I in front. The function for the validation is:

=IF(AND(LEFT(A17)="I";--MID(A17;2;6)>100000;--MID(A17;2;6)<900000);OCH(A17>100000;A17<900000))

The code I have is:

If Range("A" & i).Value > "299999" Then
If Range("F" & i).Value = "" Then
MsgBox "Indicator missing on row " & i
Application.Goto .Range("F" & i)
Exit Sub
End If
End If

It works wor the nubers without the I but it demands a vlaue in F on all I numbers and I only want it for >I299999.