Level in the header is beginner. In 2001. Download Rebol/View, the source in the gist link. Run Rebol, click console. pwd, ls, cd to the place you put the gist code. type "do %whateveryounamedthesource". Smaller than a browser i would guess.

>Smaller than a browser i would guess

You can amend max-x & max-y variables in the script to change spreadsheet size to make bigger.

For others...

The Rebol 2 binaries to run this spreadsheet script can be found here - http://www.rebol.com/download-view.html

And from the shell/command line you can also run the script straight off the internet like so:

  ./rebol http://www.rebol.org/download-a-script.r?script-name=rebocalc.r

The spreadsheet is simple to use (beginner level) but surprisingly very powerful (Rebol types, expressions & functions are available).