You will need to define the textbox value as a double:
Code:
Private Sub SpinButton1_SpinDown()
Dim dblRev As Double
dblRev = CDbl(TextBox1.Text) - 0.1
TextBox1.Text = Format(dblRev, "0.0")
End Sub
Private Sub SpinButton1_SpinUp()
Dim dblRev As Double
dblRev = CDbl(TextBox1.Text) + 0.1
TextBox1.Text = Format(dblRev, "0.0")
End Sub
Private Sub UserForm_Initialize()
TextBox1.Text = Format(CDbl("1.0"), "0.0")
End Sub