Adding 2 custom fields together

Custom field Number1 and custom field Number2 answer in custom field Number4. I just dont seem to do it yet I can do so much else

You don’t get the answer in a custom field. You program the custom theme itself to display the result of a calculation using custom field contents as inputs. See

Thanks Tut I realised that after ages trying. So I have managed to change the theme and get it to do what I want .
I am now stuck on… I need to find the gross income for the last 52 weeks(not year) and the last 4 weeks(not month) but I am not sure what table to reference in order to find that

Why not a standard P&L statement for the desired time frame?

I want to be able to reference it in the theme

You cannot. Themes display individual transactions (or statements). Reports process those transactions into various summaries.