Form Question: How to get the percent calculated?

Hello!

I have a form that is a invoice like form. I am able to get the pre-tax total, then plug in the percent tax amount (it varies by county, so it can't be a static number), then the addition of the pre-tax total plus the tax. HOWEVER, I need to also know what that percent tax total is in $ form.

For example:

BOX A - Pre-Tax Total: $10.00 (calculated by form from the total above in the form)

BOX B - Tax percent: 10% (filled in by user)

BOX C - Total including tax: $11.00 (calculated from above)

What I also need is:

BOX - Tax total: $1

Final Form:

BOX A - Pre-Tax Total: $10.00

BOX B - Tax percent: 10%

BOX C - Tax total: $1

BOX D - Total including tax: $11.00

THANKS!