Bought it, built it!  Surprisingly straight forward, thanks a) to your advice nybill, and b) the link you couldn't get to worked first time for me had a pretty detailed picture guide.

"It's a bit clicky," says the wife, so all the more reason to use it.

So, here is a PG-13 picture of the calculator in action:
https://next.thelovebug.org/index.php/s/apL8pxrX7Spd6Bc

(I managed to break my GMG instance without even knowing about it! My next project, perhaps?)