Best practice for creating a sum of a formula?

What is the best way to create a sum on multiple groups from a formula or another sum?

Sum ({IC_ADJS_JNL.QTY}, {IC_ITEM_MST.ALT_ITEMB}) * {NT_CONV_FCT.NET} - This is sitting in GF1, but I also need the sum of this on GF2.

{NT_USED_DEM.USAGE_QTY} - {@RPK_LBS} - another example sitting in GF1 but needs its sum in GF2

Hopefully I have explained this well enough. Thanks in advance for your help!