Custom Calculation Script for Fillable Forms - PDFs

I am working on creating a form in which I need to calculate ship profit. My fields are as follows:

• Our Cost
• Customer Price
• Surcharge
• Subtotal (Customer Price + Surcharge)
• Ship Profit

Here is the formula in plain english.  Can someone help me write the custom calculation please?

Subtotal - Our Cost/Subtotal

I need this to be a precentage, so I don't know if it then should be:

Subtotal minus Our Cost divided by Subtotal minus 1.

Thank you in advance for your help!  Below is a screen shot of my form as is.