Adding Subtotal, Tax and Grandtotal fields - Calculating issues

Alright... I figured out how to add the tax calculation into gravity forms..

I need this:

1. Subtotal field
2. Tax field (X %)
3.Total (inkl X %)

So I put these calculation:

On 1: Calculate all products to one amount
On 2: Subtotal Field * X(%)
On 3: Subtotal Field + Tax Field = Total field

So far, the calculation is fine..it seems to work... but in the back... it calculates all these 3 total fields together and gives me a total amount of fields 1, 2 and 3... Which is of course not right

How can I solve this problem or at least have appear the amounts as desired above...

Ideas? Thank you!

Thanks Rob, I saw that already.. but does it really help for my needs?

This is just a shortcut for the subtotal.... i dont have a problem to calculate the couple products together... my problem is that I would like to display the tax and total as an extra (total) field and the way it is setup right now, it does work, but it calculates all 3 items (subtotal+tax+total), which is not correct..

Any idea how I can get a solution here?