This is my unmodded calculator. It can add the numbers 1-4, but cannot repeat the same number twice. I plan to improve on it. Here is how you use it:

Step 1: Go to Camera 1

Step 2: Put VTOL Stick all the way up.

Step 3: Press any action group 1-4 to add them. 5-8 does nothing. Make sure you keep the numbers that you added turned on until you want to reset the calculator.

Step 4: It should show the answer now, so if you want to reset, then move the VTOL Stick back to the middle, deactivate the action groups, and then put the VTOL Stick up again. It must be in that order.

Step 5: Repeat for another Calculation!

Oh, and make sure you are in the high physics setting

P.S. Please tell me if there are any problems with the craft.
P.P.S Ignore the line of pistons to the side of the craft, forgot to remove that before uploading :)
P.P.P.S. I have made a much more compact version if you would like to check it out.