Lists, inserting rows, and showing selective data

Hi,

I'm new to this and have had a look around and can't find the specific answer to my questions, so hopefully you can help.

I'm having a few basic problems with using lists. I have set up a spreadsheet with the following coloums: refrence no., name on policy, date, A premium, B premium, taken up, total premium, and brokerage. My first problem is that i would like total premium and brokerage to be automatically worked out, but only if there is a figure put into the taken up coloum. If there is nothing in the that specific row of the taken up coloum i want total premium and brokerage to remain blank in that row.
The other small problem i have is that at the bottom of some of the lists i have included a toggle total row, but not on others, now when i go to enter data it appears in different rows or doesn't allow me to type because 'cannot modify contents of total list row'.
Basically when i add a new refrence no. i want it to add a new row for the other coloums.

Thanks for the help

WTB