Tax and Totals for Australia

Hello,

I'm currently running v1.5.6.1 and would like to change how the totals and sub-totals are calculated on the shopping cart and checkout pages. I am in Australia and we have certain requirements as to how goods and services are to be itemised in relation to taxation.

I have the "Display Prices With Tax" function set to yes so the items in the shopping cart all have tax already included. This is fine.
The sub-total itself however does not include tax which is something I would like to change to include tax.
I also would like to display the freight cost (once the customer calculates freight) and a total tax line in which only the tax amount from both the products and the shipping is itemised.
Ideally something like this at the bottom of the cart:

Sub-total (tax included)
Freight (tax included)
Tax (from products and freight)
Grand total

What would the best way be to go about setting this up and also having it correctly show up on the paypal payment/invoice?

Thanks in advance

Okay so I have changed the way the totals are to be displayed and am now looking to make a minor change with how tax is calculated.

Currently, the site shows the totals as follows:

Sub-Total (Inc. tax):
Shipping (Inc. tax):
Tax included:
Grand total:

I would like the calculation to be:
Sub-Total (Inc. tax) + Shipping (Inc. tax) = Grand total.
The tax included figure should show 10% of the subtotal + 10% of the shipping as one figure but NOT have it add to the grand total again.

Currently it is resulting in the tax being added to the grand total twice.

What would be the best way of changing this?

Could anybody at least specify which files control the calculations of the sub-totals, tax and freight charges? I've noticed that I currently am having the tax be added twice into the total.

Thanks

The files that determine the Order Totals are in:

/catalog/model/total/