Going from dynamic to fixed value in a portal
I'm designing a database with a billing section, where the main billing is done in a portal that uses a dynamic calculation (GetNthRecord function) to get the previous balance from the last bill date. This works fine, except that I also need to generate a bill based only on certain dates (such as a monthly bill), and I'm using another, filtered portal (with user-selected date ranges) to generate the bill. What happens though is that my dynamically-calculated previous balance disappears because the calc thinks that the first date in the filtered portal is the first record ever, and so assumes that there is no previous balance.
How can I grab the previous balance amount from the non-filtered portal and make it a "fixed" value? I've thought of copy and pasting into a separate field, but I wouldn't want to do that for all bill dates in a client's record because it might take a long time to do it each time (as sometimes there would be the option to go back in and change payments in previous bill dates, etc., so I would want the ability to just generate the previous balance for the first date in the filtered portal each time I create a bill).
Make sense? My convoluted grammar is indicative of the muddiness of my thinking around this in general...
I'm sure there's a very obvious solution to this, but my head's swimming in confusion at this point.
Thanks ever so...