Hi Siimont, welcome on the daniweb site.
Have a look at this thread, in one of the posts it has a zip file with some code for a calculator. It is a simulation of a calculator I have at home.
It acts differently than the MS one but if you look at the code it must be somehow possible to convert this into the action an MS calculator does in case of a percentage calculation.
This is a piece of my code that handles the "function" buttons on my calculator, for further details look in the zip file, or ask a question.
For Each ctrl As Control In Me.Controls("pnlMainPanel").Controls
If ctrl.GetType Is GetType(System.Windows.Forms.Panel) Then
For Each subCtrl As Control In ctrl.Controls
If subCtrl.GetType Is GetType(System.Windows.Forms.TextBox) Then
If subCtrl.GetType Is ...