I can think of a couple of ways to accomplish this:
1. Have a textbox control on a slide that you would type the value into.
Have a button that saves this value to some variable, then moves to the next
slide. On the desired slide, have the value of a textbox be equal to the
variable captured previously. As far as it being a "spreadsheet" design,
you might be somewhat limited. Keep in mind that PowerPoint needs to have
access to the object that you want to make equal to this value. That limits
you to what object it can be put in.
2. You could have an action button that links to Excel to easily do the
calculations. Then all you have to do is close Excel and you return to your
presentation in Slide Show mode.
If you have something that you are working on, you could zip it up and send
it directly to me and I will take a look at it (use the following link to
access my e-mail address -
http://www.pttinc.com/contact.html). Also, check
out David's examples on his web site. They might provide the impetus you
need to delve into VBA!