I think the best software to learn first is pencil and paper. Calculate a few unity formula by hand.
I use insight online, it is good but some things about it annoy me. Every time you edit or load up something it reloads the page and scrolls you all the way to the left. Only a problem when you have more than three recipes open at once but not a smooth workflow.
I like to support hanson because of the great reference database he has.