Calculations Based Columns In Entry Form

I've got a data entry form that is used to enter incoming waste consignment data. (I've attached a screen grab of the form) Each screen represents one record and the data is recorded in a table called "IncomingWaste". The top of the form (light blue) refers to the customer/producer of the waste. Most of that detail is held in another table on the db. The bottom (Dark Blue) part is used to record the consignment data. It gives the user a grid to enter the individual waste components within the consignment (choosing from Drop down menus for the various descriptive elements) Most crucially, the weight of each component is recorded.

I have a calculated field at the bottom which gives a Total weight for that consignment. It simply adds the weight fields together and delivers a total in the box at the bottom of the screen which displays the total weight in that consignment using "Nz([weight1])+Nz([weight2)]+ ...etc - which works fine on the form. However, I can't seem to get this calculated total recorded and appended to each record in the table and I assume it must be re-calculated each time the screen loads (?)

The main issues is.... I need to create a report which adds the total weights from each customer over a monthly/quarterly period and I simply cannot seem to do it.

I have created the report which groups all the consignments from each customer and lists them in date order, but I can't identify or define the method I need to use to collect that Total Weight calculation from each consignment record and add them up to give a grand total for that customer for a particular period. I'm assuming if I can crack the first part and get the total recorded in the table, then I can call the value into the report.