Excel 2003

I am developing a userform in Excel using MS Visual Basic

I would like to be able to have a form text box (?) perform a calculation
using numbers entered in previous fields, show the result of that calculation
immediately for visual confirmation and then enter that result in the proper
field in the database.
Example... enter the following into form fields:
Contract Cost: (enter no.)
Hard Costs: (enter no.)
Mark Up: (enter no.)
Gross Margin %: (formula; based on Markup/Contract Cost) (View this data
immediately on the form and then upon submitting form entering calculation in
GM% column with this project.

Also are there any limitations as to how many fields I can enter onto a
custom userform?

Thanks for any help!