Running subtotal of previous record for each current record

[Todd H]

I have created a report for a Contract Change Order. Each Contract
Change Order is made up of of several (one to many) individual
'changes.' I have the report compiled with all the pertinent
information, and everything set up and working. I have one problem, I
need to create a code expression to create a subtotal sum of all the
Previous Contract Change Orders for each 'current' Change Order. So
the summary total is in a format for each Change Order: Original
Contract Amount + Previous Contract Change Orders + (Current) Contract
Change Order = Current Contract Amount. For instance, if Contract
Change Order No. 3 was issued, the Previous Contract Change Orders
would be the sum of Change Order 1 & 2; Change Order No. 4's Previous
Change Orders would equal the sum of Change Orders 1, 2, 3...and so
on.

So here are the specifics; here is the code from my query I am pulling
all this from:

SELECT qryFeldOrderSubFormNew.PTCContractID,
qryFeldOrderSubFormNew.ContractDate,
qryFeldOrderSubFormNew.OrigContract, tblContacts.CompanyName,
tblContacts.Address, tblContacts.[City] & ", " & [StateOrProvince] & "
" & [PostalCode] AS CityStateZip, qryFeldOrderSubFormNew.PCCO_No,
qryFeldOrderSubFormNew.PCCODate, qryFeldOrderSubFormNew.ID,
qryFeldOrderSubFormNew.PTCCODescription,
qryFeldOrderSubFormNew.PCOFinalCost,
qryFeldOrderSubFormNew.PCOApprovedDays, tblProjectInfo.ProjNo,
tblProjectInfo.ProjNameLong
FROM tblProjectInfo, (qryFeldOrderSubFormNew INNER JOIN tblContacts ON
qryFeldOrderSubFormNew.PTCResp = tblContacts.CoNameShort) INNER JOIN
tblSubContacts ON tblContacts.ContactID = tblSubContacts.ContactID
GROUP BY qryFeldOrderSubFormNew.PTCContractID,
qryFeldOrderSubFormNew.ContractDate,
qryFeldOrderSubFormNew.OrigContract, tblContacts.CompanyName,
tblContacts.Address, tblContacts.[City] & ", " & [StateOrProvince] & "
" & [PostalCode], qryFeldOrderSubFormNew.PCCO_No,
qryFeldOrderSubFormNew.PCCODate, qryFeldOrderSubFormNew.ID,
qryFeldOrderSubFormNew.PTCCODescription,
qryFeldOrderSubFormNew.PCOFinalCost,
qryFeldOrderSubFormNew.PCOApprovedDays, tblProjectInfo.ProjNo,
tblProjectInfo.ProjNameLong
HAVING (((qryFeldOrderSubFormNew.PCCO_No)<>0));

The Contract Change Order totals are the
Sum(qryFieldOrderSubFormNew.PCOFinalCost)
The Contract Change Order Numbers are qryFieldOrderSubFormNew.PCCO_No
Each Contract is identified by qryFieldOrderSubFormNew.PTCContractID

So I need to create a calculated variable of the total of all the
previous Change Orders based on the current
qryFieldOrderSubFormNew.PCCO_No. The report is based on each
Contract, and the report will be run to include all change orders per
contract, which means that the variable will need to change for each
qryFieldOrderSubFormNew.PCCO_No, and that could be several times when
I execute the report.

I hope I have explained by problem clearly, and I appreciate any help
you all can give. I'm sure this is not an isolated request, but I
couldn't get the wording correct to find anything online to help me
along.

Thanks!
Todd