Shipping to include tax

[mcpisik]

Hi,
I have tax calculating on shipping, but I'd like it to automatically calculate the tax.

eg.
At the moment I need to type in $10.43 and it will then add 15% to make $11.99 (aiming for a nice rounded $12 as well)
I would like to set it at $7 and have it calculate the amount for me.
(I have variable shipping that I quote for and create the orders myself very often each day)

When creating products I can type in the gross field, but not sure where to do that for this?
Thanks and sorry if it's obvious, but first time adding taxes.