I've seen some demo applications that use a webviewer to display temproal information such as a calculation of found records vs. total.

How is a web viewer supposed to be stup in order to disply such data?

For example, if I enter a Let function which calculates a subtraction between two fields, how can I display the result in order to avoid creating an extra field in the table?

It can get fancier, including formatting, but here's a start:

Let( myTotal = thisField + thatField;

"data:text/html," & myTotal

)

Great! that starting data html text was the key to my problem...

I'll add HTML in my things-to-learn list :)