Crystal Reports Multiplying rolling totals?

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     Hi, I've just recently started testing the waters a bit with Crystal Reports and have encountered what should be a pretty simple problem.  Given a database with multiple tables, how would it be possible to have a formula field take into account the previous calculated value?  I tried testing around with the rolling totals, but most of the functions weren't right for this purpose.  A ficticious example would be as follows:

Imagine if you had $1,000,000 invested somewhere and you had monthly returns, how would you calculate the month-end account value?  If let's say

Month                     Return                    Value

Jan                          5%                (1.05*1000000) = $1050000
Feb                         -2%               (0.98*1050000) = $1029000
Mar                         8%                (1.08*1029000) = $1111320

etc.

Thanks in advance