RPN PopUp Calculator 1.0.1
the author of this app says:
RPN PopUp Calculator is a HackMaster extension which pops up a calculator over the active application. It is activated with the simple (definable) pen stroke. When you are finished with calculation, you are back to your application with just one tap. You can exchange data with your application if it supports copy and paste, or if it has an input field. Numbers and operations can also be entered with graffiti. RPN PopUp Calculator has a simple forth-like script language, so you can write your own functions.