Lack of indexing does not prevent performing finds. It just takes longer. As long as this is a field of type calculation and not a field of type summary, you can enter find mode and specify criteria in the field.
If that does not work fast enough for you, you can use a simple number field with script triggers that update the field via script each time you edit a field and record that is part of the 'sum' this field is intended to report.
See this link for a very similar discussion of this concept: Searching a summary field.
Phil, I have uploaded a screen shot of my "ledgers" page.
This page has a portal which the amount paid gets recorded. The resulting ongoing balance is different every month since my patients are on monthly payment plans. The Due Today is the amount due this month. when I preform a find on it , it returns incorrect results. If I select the field and >0 to find patients with balances, it returns no records match and there are several with positive balance.
There are two ways to compute a total for a portal:
One is to use a summary field defined in the portal's table.
The other is to use a calculation field defined in the parent table that uses Sum to total the field from the related set of records.
If you use the second option, you should be able to use it in finds performed on the parent table.