A portal means that you have a one to many relationship:
If you define a calculation field in the LayoutTable like this:
Sum ( PortalTable::FieldToSumHere )
You will compute your total over all the related records in the portal, IF it is an unfiltered portal. If you define a portal filter, duplicate your portal (so that it has the same filter expression), reduce the number of rows to just one row and put a summary field defined in the portal table to compute this same total into that single portal row.
Other options are also possible.
Second one works great as I have filtered portals.
Thanks once again for your help :)