Formula for decreasing value.

Okay, Try this UDF solution, will work for both scenarios

Function CutBoard(StrtLeng, FrstCutLeng, CutIncr, Optional NumCutsPer, Optional IncldCurf, Optional CurfSz, Optional ReturnOpt)
    Dim LengthRemain, LastCut, NewCutLen
    Dim i, TotalCuts As Integer
    Dim ErrMess As String
    Dim EXIT_FLAG As Boolean
    If IsMissing(NumCutsPer) Then NumCutsPer = 1
    If IsMissing(IncldCurf) Then IncldCurf = 0
    If IsMissing(CurfSz) Then CurfSz = 1 / 8
    If IsMissing(ReturnOpt) Then ReturnOpt = 0
    ErrMess = ""
    If StrtLeng <= 0 Then
        ErrMess = "Invalid Starting Length"
    ElseIf FrstCutLeng <= 0 Or FrstCutLen > StrtLeng Then
        ErrMess = "Invalid First Cut Length"
    ElseIf CutIncr <= 0 Or CutIncr > FrstCutLeng Then
        ErrMess = "Invalid Cut Increment"
    ElseIf NumCutsPer < 1 Then
        ErrMess = "Cuts per cut length MUST be at Least 1"
    ElseIf IncldCurf <> 1 And IncldCurf <> 0 Then
        ErrMess = "Include Curf is not 1 or 0"
    ElseIf ReturnOpt < 0 Or ReturnOpt > 3 Then
        ErrMess = "Return Option MUST be 0,1,2 or 3"
    End If
    If ErrMess <> "" Then
        CutBoard = ErrMess
        Exit Function
    End If
    EXIT_FLAG = False
    TotalCuts = 0
    LengthRemain = StrtLeng
    NewCutLen = FrstCutLeng
    While LengthRemain > 0 And NewCutLen > 0 And EXIT_FLAG = False
    
        LastCut = NewCutLen
        If LengthRemain - (NewCutLen * NumCutsPer) > 0 Then
            LengthRemain = LengthRemain - (NewCutLen * NumCutsPer) - (IncldCurf * CurfSz * NumCutsPer)
            NewCutLen = NewCutLen - CutIncr
            TotalCuts = TotalCuts + NumCutsPer
        Else
            EXIT_FLAG = True
            For i = 1 To NumCutsPer
                If LengthRemain - NewCutLen > 0 Then
                    LengthRemain = LengthRemain - NewCutLen - (IncldCurf * CurfSz)
                    TotalCuts = TotalCuts + 1
                End If
            Next i
        End If
    Wend
    Select Case ReturnOpt
        Case 0
            CutBoard = TotalCuts
        Case 1
            CutBoard = LengthRemain
        Case 2
            CutBoard = LastCut
        Case 3
            CutBoard = "Total cuts : " & TotalCuts & " Last Cut Was : " & LastCut & " Length Remaining : " & LengthRemain
    End Select
End Function

to use, allow Macros,open VBA editor (Alt-F11), Insert Module, Copy & paste above into the module,save close editor

Parameters are
StrtLeng, FrstCutLeng, CutIncr, Optional NumCutsPer, Optional IncldCurf, Optional CurfSz, Optional ReturnOpt
StrtLeng - Starting length of your board, in decimal inches (ie 100 1/2 inches = 100.5)
FrstCutLeng - Length of your first cut(s) in decimal inches
CutIncr - length to decrease cut length by in decimal inches
NumCutsPer (Optional) - how many cuts to make at each Length (defaults to 1)
IncldCurf (Optional) - 1 = include saw curf,0 = don't include saw curf (defaults to 0)
CurfSz (Optional) - size of the saw blade curf in decimal inches (defaults to 0.125 (1/8"))
ReturnOptions (optional) 0- Total number of cuts made, 1- length of board left (if any) after as many cuts can be made using first 4 parameters, 2- the length of the last cut, in decimal inches, 3- a brief text summary of options 1,2&3 (defaults to 0)

See attachment for examples

Hope this helps