Hey Phil -
Sounds like a fun project. I think there is an opportunity here to actually be more efficient than QB, since you can create a custom calcuator for their unique business needs - just might require some creativity. I know you are going to get to the point where the popup gets the value and you want to put it on the original screen when the popup closes.
What you want to do is set a SharedResult on the popup when it closes. And then on the original screen, edit the screen, click Advanced, go to the OnPopUpClose event and use that to grab the SharedResult and put it into the grid. For an example to reverse engineer, have a look at the Opportunites screen and what happens when you click the "..." on the customer, add a new customer and have it pop in when the popup closes.
Calculator - I can see us doing this one day. Personally when I use QuickBooks I use the calculator all the time. It's no small thing to do on a web page, though. But, hey, we've taken on more challenging projects than that for Method.
I don't see us adding a calculator any time soon. We've got lots of goodies on the go, some even coming out on next week's version - which I plan to write a geeky blog post about. We also have some bigger projects too that are occupying a dev team's time.....so I honestly don't see a calculator finding its way up the priority list for at least the next few months.
Paul