Simple Calculation Scripts

I am new to Adobe and creation of forms and I am just looking to create a calculation for the following scenario:

I am creating a sales commission form and i have the following text boxes:

Total Commission to be paid [________] --> this value will be entered

Payable on Firm [________] --> I want this to be calculated as (total commission to be paid / 2)

Payable on Close [_________]--> I want this to be calculated as (total commission to be paid / 2)

I know how to create a calculation when doing the product of two text boxes, but when i need to just calculate a text box by a specific numeric value I am lost as to how this script would look. I have other forms under construction with similar scripts needed ie. pricing with a subtotal that must be multiplied by HST (0.13) to get a total etc.

Any help would be greatly appreciated.

Thanks