Calculation Documentation

Is there not any documentation for how to use Calculation? I have one field in my form where the user enters a number. Then there are a few product fields where I want that number to be multiplied by the price to automatically generate the price for that item. So if they enter 2 up above, then the product will calculate the price 2 x $35 and charge then $70. (This is for a conference registration, and the number they enter changes the options for several different things, and I don't want to have to keep asking them for that same number over and over.) I thought I could do it with a calculation, but it doesn't seem to be working, and I can't find any documentation.

Thanks for that link! It is working now. Gravity Forms seems a little buggy since the last upgrade. I get stuck where I can't edit anything, have to update form, leave, come back and try again. Sometimes I have to entirely create a new field and start over when I can't edit one. Anyway, I now have the calculation fields working, thanks!