Do you have accurate specs and drawings of your runner system? If so, it should be a relatively simple matter to calculate the runner volume and figure a weight. If not, I don't know of a standard "weight adder" calculation for the runner based solely on part weight. The part configuration and tool configuration both influence the end runner design. Even when we knew our runner volume, it only gave us a starting point. We always had to zero our shot in by trial and error. I do consulting in this area. If you're interested in some detailed help, PM me.

I'm a die cast engineer and I have to do that calculation all the time. First, do you have a mold (tool) drawing? Measure the surface area, add in the avg. wall thickness + runner and overflows. Get cubic inches. Use the spacific weight of the material being run. That will give you shot weight.

usually it is unimportant -- they are either trimmed on end of robot or by hand and reground and put back into the hopper for the next shot

after they are filled - it is also not important in sizing the machine for the part/ shot size

best of luck