how do i keep the calculator on top?

Here is a free app that I believe needs simbl to work, its called:

Afloat

(http://infinite-labs.net/afloat/)

Afloat offers "window management features," to apps.

I do not know if it will work with Calculator.

Otherwise, here is a .99¢ alternative:

Always on Top Calculator 4.5

(http://itunes.apple.com/us/app/always-on-top-calculator/id491093921?mt=12))

"Always on Top Calculator," comes in a standard algebraic version and an RPN version.

Don't run the spreadsheet or webpage in full-screen mode. This will allow the calculator to be kept visible off to one side.

If the calculator stayed on top, meaning being the active program, how would it know when you wanted to enter numbers in the spreadsheet rather than in the calculator?

If you keep the calculator off to one side, then all you need do is click its frame to make it active.

Not sure but Command+Tab will bring it to the front when needed.