You will only be able to reference the total of the cart for a discount or charge rule, not in a calculation. This is because calculations are performed on a per item basis with values for an individual item, not the whole cart.
If you have a special charge, discount, shipping or tax rule you can make use of the cart's sub total by selecting 'subtotal of cart' from the select lists. If you are custom coding the rules you can specify the sub total of the cart like this:
The tax can be specified in the code like this:
You would need to use the name of your cart in place of eCart1. If you have any further trouble with this post back with some more details about how your rules and calculations are currently setup and how you would like them to work in the end.