Get closest cell with a value before the current - excel

Question

In a column filled mostly by blank cells, I need to get the value of the cell thats closest to the currently selected one, in a row before it, and isn't blank.
While the "isn't blank" part can be easily achieved with the an IF and ISBLANK statement, I can't figure out how to get the position of the first cell upwards from the current one that isn't blank.
In this example spreadsheet, in the cell C7 I want to display the value of B7 minus the value of B4 (=B7-B4). The problem is that these values are separated by an unpredictable number of cells, which can range from 0 to 20 or even more. Needing to parse a spreadsheet with thousands of rows and frequent data added, selecting the upper cell manually is not an option.
In continuation of my example, I want C13 to display =B13-B10, but lets say there were two more blank rows in between them (row 13 becomes row 15), I would want it to display =B15-B10.
Can the position be obtained with a MATCH query, knowing that the Types will always be T1 or T2?
Thank you in advance.

Answer 1

Enter this formula in cell C2 and drag it to the bottom.
=IFERROR(IF(B2="","",B2-LOOKUP(9.99999999999999E+307,B$1:B1)),"")

Answer 2

Writing formulae via VBA
Below you find a VBA approach writing your subtracting formulae to column C.
BTW, searching via array is much more quicker than looping through a range.
Code
Option Explicit
Sub subtractLastValueRow()
' declare vars
Dim oSht As Worksheet ' work sheet
Dim a As Variant ' one based 2-dim data field array
Dim n As Long ' last row in column B
Dim i As Long ' item no
Dim ii As Long ' last item no
Dim j As Long
Dim s As String
' set sheet
Set oSht = ThisWorkbook.Worksheets("MySheet") ' fully qualified reference to worksheet
' get last row number of search column
n = oSht.Range("B" & oSht.Rows.Count).End(xlUp).Row
If n < 2 Then Exit Sub ' only if data avaible (row 1 = title line)
' get range (after title line) values to one based 2dim data field array
a = oSht.Range("B2:C" & n).Value ' array gets data from e.g. "A2:A100"
' loop through column B to find keyword sKey
If Len(a(1, 1) & "") > 0 Then ii = 1 + 1 ' first item in array
For i = LBound(a) + 1 To UBound(a) ' array boundaries counting from 1+1 to n -1 (one off for title line)
' value found
If Len(a(i, 1) & "") > 0 Then
For j = i + 1 To UBound(a)
If Len(a(j, 1) & "") > 0 Then
' write .Formula (alternatively use .Value, if value wanted)
oSht.Range("C" & i + 1).Formula = "=B" & i + 1 & "-B" & ii
ii = i + 1 ' note last found i
Exit For
End If
Next j
End If
Next
If Len(a(UBound(a), 1) & "") > 0 Then ' last item in array
oSht.Range("C" & UBound(a) + 1).Formula = "=B" & UBound(a) + 1 & "-B" & ii
End If
End Sub
Note
If you want to write values instead of work sheet formulae, simply replace .Formula with .Value.

Answer 3

In cell C3 put following formula:
=B3-LOOKUP(2,1/$B$1:B2,$B$1:B2)
And copy down in all T2 cells as required.